Skip to main content

Full text of "The manufacture of mineral and lake pigments, containing directions for the manufacture of all artificial artists' and painters' colours, enamel colours, soot and metallic pigments"

See other formats


THE MANUFACTUEE OF MINEEAL AND 
LAKE PIGMENTS 



THE MANUFACTURE OF 

MINERAL AND LAKE PIGMENTS 



CONTAINING DIRECTIONS FOR THE MANUFACTURE 

OF ALL, ARTIFICIAL ARTISTS' AND PAINTERS' COLOURS, 

ENAMEL COLOURS, SOOT AND METALLIC PIGMENTS 



Ji :TeHSooft for 
4nanti?actftrers, 4ttercflSnts t Jirtists and Painters 

BY 

DR. JOSEF BERSCH 

il 
TRANSLATED FROM THE SECOND, REVISED EDITION 

BY 

AETHUK C. WEIGHT, M.A. (OxoN.), B.Sc. (LoND.) 
BOOK. 



The Publishers wish it to be distinctly understood that this 
book is supplied on such terms as prohibit it being sold below the 
published price. 

SGOTT, GREENWOOD & Go. 



LONDON 
SCOTT, GKEENWOOD & CO. 

19 LUDGATE HILL, E.G. 
1901 

"[The sole right of publishing this work in English rests with the above firm.] 




THE MANUFACTURE OF 

MINERAL AND LAKE PIGMENTS 



CONTAINING DIRECTIONS FOR THE MANUFACTURE 

OF ALL ARTIFICIAL ARTISTS' AND PAINTERS' COLOURS, 

ENAMEL COLOURS, SOOT AND METALLIC PIGMENTS 



Ji 3"et*Sooft for 
, 4ttercttSnts t Artists Sn6 ^Sinters 



BY 

DR. JOSEF BERSCH 

TRANSLATED FROM THE SECOND, REVISED EDITION 
BY 

AKTHUK C. WEIGHT, M.A. (Oxou.), B.Sc. (LoND.) 

FORMERLY ASSISTANT LECTURER AND DEMONSTRATOR IN CHEMISTRY AT THE 
YORKSHIRE COLLEGE, LEEDS 



WITH FORTY-THREE ILLUSTRATIONS IN THE TEXT 



LONDON 

SCOTT, GEEENWOOD & CO. 

\ BRA ftXv rpufifts^erfi of i$t ++ dtf an?> ofourman + 6 

(\9 THE 



UNtYER8(TY 

OF 1901 



[The sole right of publishing this work in English rests with the above firm.] 



PREFACE. 

WE know hardly another branch of chemical techno- 
logy which has made such remarkable advances of 
late as the manufacture of colours ; a large number of 
pigments have been recently discovered, distinguished 
by beauty of shade and permanence. Chemists are 
continually endeavouring to replace handsome and 
poisonous colours by others equally handsome but 
non-poisonous. 

In writing this work I have endeavoured to give 
it such a character that it may be a text-book for the 
practical man, only those methods have been given 
which certainly lead to a good result ; in the case of 
new pigments I have only described methods of pre- 
paration which I have myself found to give good 
results. 

Since it cannot be imagined that any one quite 
ignorant of chemistry could successfully manufacture 
colours (colours being always made by chemical pro- 
cesses which occasionally are rather complicated), I 
have, therefore, presupposed a knowledge of the prin- 
ciples of chemistry. In the short sketch of the 
chemical properties of the raw materials used in mak- 
ing ordinary pigments, the principal properties of the 
materials used by the colour-maker are given. 



145138 



VI PEEFACE. 

In order to make this treatise useful to dealers in 
and consumers of colours, the chapters dealing with 
the examination of pigments have been so arranged 
that the nature or adulteration of a pigment can be 
determined quickly and with certainty by any one. 

Recipes, which originated at a time when empiri- 
cism ruled in chemistry, have been omitted, since 
they would only detract from the clearness of the 
matter. 

As far as it is possible I have avoided the " recipe 
fetish," and have endeavoured to make clear to the 
reader the chemical processes to which regard must 
be had in the manufacture of the different pigments. 
Since the appearance of the first edition there have 
been many valuable innovations in the mineral colour 
industry, to which regard has been given in preparing 
this second edition in so far as they possess a really 
practical value. 

A critical examination of proposals and formulae, 
which are found in large numbers in the journals, has 
been avoided, since I wished to keep for my book 
that character of a reliable text-book and book of 
reference which was ascribed to it in the form of its 
first edition. 

DR. JOSEF BERSCH. 



TRANSLATORS PREFACE. 

IF excuse be needed for presenting a translation of 
Dr. Bersch's book at so long an interval after the 
publication of the original (1893), it must be sought 
in the paucity of the English literature on the subject. 
It is hoped that the practical nature of the work 
will make it acceptable to the English reader. 

The subject-matter of the original has been pre- 
served in the translation without alteration or 
addition, with the exception of an unimportant 
change in the order of arrangement. 

The metric system of weights and measures has 
been used throughout ; for the convenience of those 
who are not familiar with this system, directions are 
given in an appendix for converting into English 
weights and measures. 

The section on paint grinding (Chapter LXIX.) 
is perhaps somewhat incomplete ; for a more detailed 
and modern account of this branch of the subject 
the reader is referred to Practical Paint Grinding, 
by Mr. J. Cruickshank Smith, B.Sc., shortly to be 
issued by the same publishers. 

A. C. WEIGHT. 

HULL, January, 1901. 



TABLE OF CONTENTS. 

PAGE 

'CHAPTER I. INTRODUCTION . 1 

CHAPTER II. THE PHYSICO-CHEMICAL BEHAVIOUR OF PIGMENTS . . 8 

CHAPTER III. RAW MATERIALS EMPLOYED IN THE MANUFACTURE OF 

PIGMENTS 14 

HAPTER IV. ASSISTANT MATERIALS 16 

Water Chlorine Ammonia The Hydrometer Sal Ammoniac or 

Ammonium Chloride Ammonium Sulphide. 
Acids. Hydrochloric Acid Sulphuretted Hydrogen Sulphuric Acid : 

Oil of Vitriol, Nordhausen Sulphuric Acid Nitric Acid Aqua 

Eegia Carbon Carbonic Acid Gas. 
Organic Acids. Acetic Acid Oxalic Acid Tartaric Acid. 

CHAPTER V. METALLIC COMPOUNDS 32 

Alkalis. Potassium Compounds Potassium Carbonate Potassium 
Hydroxide Potassium Nitrate Potassium Bitartrate Potassium 
Bichromate Potassium Sodium Chromate Chrome Alum 
Potassium Ferrocyanide Potassium Ferricyanide Sodium 
Salts Sodium Carbonate Sodium Hydroxide Sodium Thio- 
sulphate Sodium Chloride Salts of the Alkaline Earth Metals. 

'Calcium Compounds. Calcium Oxide Calcium Hydroxide Calcium 
Carbonate Calcium Sulphate Calcium Phosphate Magnesium 
Carbonate Barium Compounds Barium Chloride. 

Aluminium Compounds Aluminium Sulphate. 

The Alums. Potassium Aluminium Alum Roman Alum Soda 
Alum Ammonia Alum Alumina and Hydrate of Alumina. 

^Compounds of the Heavy Metals. Zinc Compounds Cadmium 
Compounds Iron Compounds Ferrous Sulphate Ferrous Chlo- 
ride Manganese Compounds Nickel Compounds Cobalt Com- 
pounds Chromium Compounds Molybdenum, Tungsten and 
Vanadium Compounds Antimony Compounds Bismuth Com- 
pounds Tin Compounds Arsenic Compounds Lead Compounds 
Lead Sulphate Lead Nitrate Lead Acetate Basic Lead 



X TABLE OF CONTENTS. 

PAGE 

Acetate Lead Chloride Copper Compounds Copper Sulphate 
Copper Nitrate Copper Acetate Mercury Compounds Mercurous 
Nitrate Mercuric Nitrate Mercurous Chloride Mercuric 
Chloride Silver Compounds Gold Compounds. 

CHAPTER VI. THE MANUFACTURE OF MINERAL PIGMENTS . . .71 
White Mineral Pigments White Lead. 

CHAPTER VII. THE MANUFACTURE OF WHITE LEAD . . . .77 
Manufacture of White Lead from Metallic Lead. Dutch Process: 
Casting the Lead into Sheets Building up the Stacks Kemoval 
and Grinding of the White Lead White Lead Mills Hard White 
Lead Soft White Lead. German Process. French Process : 
Preparation of the Solution of Basic Lead Acetate Preparation of 
the Carbonic Acid and Precipitation of the White Lead Manu- 
facture of White Lead by Means of Natural Carbonic Acid. English 
Process. Other Methods. Oxychloride White Lead: Lead Sulphite 
Lewis & Bartlett's White Lead Pigment. White Lead- Antimony 
Pigments : Lead Antimonite Lead Antimonate. 

CHAPTER VIII. ENAMEL WHITE 116 

Lithopone. 

CHAPTER IX. WASHING APPARATUS 120 

Filter Presses. 

CHAPTER X. ZINC WHITE . . 126 

Griffith's Zinc White Tungsten White. White Antimony Pigments : 
Antimony Trioxide Antimony Oxychloride Bismuth White Tin 
White Manganese White Magnesia White or Mineral White 
Annaline. 

CHAPTER XI. YELLOW MINERAL PIGMENTS 133 

CHAPTER XII. CHROME YELLOWS 134 

Lead Chrome Yellow Preparation of the Lead Solution Precipitation 
of the Chrome Yellow The Pale Chrome Yellows. 

CHAPTER XIII. LEAD OXIDE PIGMENTS 143 

Red Lead, Minium. 

CHAPTER XIV. OTHER YELLOW PIGMENTS 148 

Cassel Yellow Montpellier Yellow Turner's Yellow or English 
Yellow Naples Yellow Antimony Yellow Calcium Chrome 
Yellow Barium Yellow, Yellow Ultramarine or Permanent Yellow 
Zinc Chrome Yellow Cadmium Chrome Yellow Cadmium 



TABLE OF CONTENTS. XI 

PAGE 

Yellow Lead Iodide Mars Yellow Siderin Yellow Aureolin 
Tungsten Yellow Nickel Yellow Mercury Yellow or Turpeth 
Mineral Yellow Arsenic Pigments Lead Arsenite Thallium 
Pigments. 

CHAPTER XV. MOSAIC GOLD 160 

Chrysean. 

CHAPTER XVI. RED MINERAL PIGMENTS 163 

Vermilion Black Mercuric Sulphide Red Mercuric Sulphide. 

CHAPTER XVII. THE MANUFACTURE OP VERMILION 166 

Dry Method : Chinese Vermilion. Wet Method : Firmenich's Method 
Liebig's Process Infusible White Precipitate Electrolytic Pro- 
cess Mercuric Iodide. 

CHAPTER XVIII. ANTIMONY VERMILION 178 

Appendix Antimony Blue. 

CHAPTER XIX. FERRIC OXIDE PIGMENTS 180 

Vogel's Iron Red Macay's English Red Indian Red. 

CHAPTER XX. OTHER RED MINERAL PIGMENTS 186 

Chrome Red or Chrome Vermilion Cobalt Red Cobalt Magnesia 
Red Cobalt Arsenate Chromium Stannate Silver Chromate. 

CHAPTER XXI. PURPLE OP CASSIUS 190 

Magnesia Gold Purple Alumina Gold Purple. 

CHAPTER XXII. BLUE MINERAL PIGMENTS IRON-CYANOGEN PIGMENTS . 194 
Chinese Blue Prussian Blue Mineral Blue Soluble Prussian 
Blue Special Processes for the Manufacture of Chinese Blue 
Turnbull's Blue Antwerp Blue. 

CHAPTER XXIII. ULTRAMARINE 204 

CHAPTER XXIV. THE MANUFACTURE OF ULTRAMARINE .... 211 
Preparation of Mixtures for Ultramarine Ultramarine Violet 
Chlorine and Steam Process Hydrochloric Acid and Air Process 
Ammonium Chloride Process Pale Blue Ultramarine Ultra- 
marine Red. 

CHAPTER XXV. BLUE COPPER PIGMENTS 226 

Bremen Blue and Green Neuberg Blue Lime Blue Payen's Moun- 
tain Blue Oil Blue Copper Hydroxide. 

CHAPTER XXVI. BLUE GOBALTJPIGMENTS 230- 

Cobalt Blue, Thenard's Blue, Cobalt Ultramarine, King's Blue, Leyden 
Blue Cseruleum Cobalt Zinc Phosphate. 



Xll TABLE OF CONTENTS. 

PAGE 

CHAPTER XXVII. SMALTS 233 

Preparation of the Charge Fusion of the Charge Grinding the Fused 
Mass Tungsten Blue Tessie du Motay's Blue Molybdenum 
Blue. 

CHAPTER XXVIII. GREEN MIN ERAL _Pio MENTS GREEN COPPER PIG- 
MENTS ' 240 

Green Copper Pigments : Copper Carbonate Copper Arsenite 
Scheele's Green Swedish Green Brunswick Green Green 
Verditer Neuwied Green Copper Oxychloride. 

HAPTER XXIX. EMERALD GREEN 244 

Manufacture of Emerald Green from Verdigris Manufacture of 
Emerald Green from Copper Sulphate^ Mitis Green or Vienna 
Green Copper Stannate Kuhlmann's Green Eisner's Green 
Casselmann's Green Lime Green Patent Green Copper Bor- 
ate Copper Silicate (Egyptian Blue). 

CHAPTER XXX. VERDIGRIS 252 

Blue Verdigris Distilled or Crystallised Verdigris German Verdigris. 

CHAPTER XXXI. CHROMIUM OXIDE 260 

Chrome Green. 

CHAPTER XXXII. OTHER GREEN CHROMIUM PIGMENTS . . . .264 
Guignet's Green Emerald Green Chrome Green Lake Turkish 
Green Leaf Green. Chromium Phosphate Pigments : Arnaudan's 
Green Plessy's Green Schnitzer's Green Chromaventurine 
Chrome Blue (Gamier). 

CHAPTER XXXIII. GREEN COBALT PIGMENTS 268 

Cobalt Green. 

CHAPTER XXXIV. GREEN MANGANESE PIGMENTS 270 

Manganese Green Rosenstiehl's Green Bottger's Barium Green 
Manganous Oxide Manganese Blue. 

CHAPTER XXXV. COMPOUNDED GREEN PIGMENTS 273 

Chrome Green Eisner's Chrome Green Silk Green Natural Green 
Non-arsenical Green. 

CHAPTER XXXVI. VJOLE^MINERAL PIGMENTS 276 

Chromic Chloride Manganese Violet Tin Violet, Mineral Lake 
Copper Violet, Guyard's Violet. 

-CHAPTER XXXVII. BROWN MINERAL PIGMENTS 279 

Lead Brown Manganese Brown Pyrolusite Brown Prussian Brown 
Iron Brown Copper Brown Chrome Brown Cobalt Brown. 



TABLE OF CONTENTS. xiii 

PAGE 

CHAPTER XXXVIII. BROWN DECOMPOSITION PRODUCTS .... 283- 
Humins Bistre. 

CHAPTER XXXIX. BLACK PIGMENTS 285 

Charcoal Blacks : True Charcoal Black Vine Black Vine Black from 
Wine Lees Vine Black from Pressed Grapes Bone Black or 
Ivory Black. 

CHAPTER XL. MANUFACTURE OF SOOT PIGMENTS 294 

The Manufacture of Soot Blacks on the Large Scale. 

CHAPTER XLI. MANUFACTURE OF LAMP BLACK 307 

Calcination of the Soot Pine Black. 

CHAPTER XLII. THE MANUFACTURE OF SOOT BLACK WITHOUT CHAMBERS . 31S 

CHAPTER XLIII. INDIAN INK 31& 

Neutral Tint Black Appendix: Black Mineral Pigments Chrome 
Copper Black Chrome Black. 

CHAPTER XLIV. ENAMEL COLOURS 319' 

White Enamels Coloured Enamels : Yellow Enamels Red Enamel 
Blue Enamels Green Enamel Violet Enamel Black Enamel. 

CHAPTER XLV. METALLIC PIGMENTS 326 

Shell-Gold Shell-Silver Imitation Silver. 

CHAPTER XL VI. BRONZE PIGMENTS 329 

Electrolytic Copper Bronze Tungsten Bronze Pigments. 

CHAPTER XLVII. VEGETABLE BRONZE PIGMENTS 339 

Appendix : The Brocade Pigments. 



PIGMENTS OF ORGANIC ORIGIN. 
CHAPTER XLVIII. LAKES 343 

CHAPTER XLIX. YELLOW LAKES 348 

Dutch Pink Weld Lake Gamboge Lake Prepared Gamboge Fustic 
Lake Quercitron Lake Purree or Indian Yellow The Colouring 
Matter of Saffron Colouring Matter of Gardinia Grandiflora. 

CHAPTER L. RED LAKES 354 

Cochineal and Carmine 

CHAPTER LI. THE MANUFACTURE OF CARMINE . ... 357 

Cenette's Method Munich, Vienna, Paris or Florentine Lake Am- 
monia-Cochineal. 



XIV TABLE OF CONTENTS. 

PAGE 

CHAPTER LII. THE COLOURING MATTER OF LAC 363 

Lac Dye. 

CHAPTER LIII. SAFFLOWER OR CARTHAMINE RED 366 

Safflower Carmine Alkanet. 

CHAPTER LIV. MADDER AND ITS COLOURING MATTERS .... 370 
Garancin Garanceux Madder Extract The Constituents of Madder. 

CHAPTER LV. MADDER LAKES 375 

Madder Carmine. 

CHAPTER LVI. MANJIT (INDIAN MADDER) 378 

Chica Red, Curucuru, Carajuru Bigonia Chica. 

CHAPTER LVII. LICHEN COLOURING MATTERS 380 

Archil French Purple Cudbear Litmus. 

CHAPTER LVIII. RED WOOD LAKES 384 

CHAPTER LIX. THE COLOURING MATTERS OF SANDALWOOD AND OTHER 

DYE-WOODS . . 388 

CHAPTER LX. BLUE LAKES 390 

Indigo The Constituents of Indigo. 

CHAPTER LXI. INDIGO CARMINE 394 

Indigo Mills Blue Lake. 

CHAPTER LXII. THE COLOURING MATTER OF LOGWOOD .... 398 
Logwood Extract Kohlrausch's Process for Obtaining Concentrated 
Extracts of Colouring Matters and Tannins. 

CHAPTER LXIII. GREEN LAKES 409 

Chlorophyll Sap Green Chinese Green, Lokao Charvin's Green. 

CHAPTER LXIV. BROWN ORGANIC PIGMENTS 414 

Asphaltum Sepia. 

CHAPTER LXV. SAP COLOURS 416 

CHAPTER LXVI. WATER COLOURS 419 

Moist Water Colours. 

CHAPTER LXVII. CRAYONS 423 

Crayons for Earthenware. 

CHAPTER LXVIII. CONFECTIONERY COLOURS 427 

CHAPTER LXIX. THE PREPARATION OF PIGMENTS FOR PAINTING . . 429 
Paint Mills. 



TABLE OF CONTENTS. XV 

PAGE 

CHAPTER LXX. THE EXAMINATION OF PIGMENTS 434 

Mineral Pigments Examination with the Blowpipe Reactions of the 
White Pigments Reactions of the Yellow Pigments Reactions 
of the Red Pigments Reactions of the Blue Pigments Reactions 
of the Green Pigments Reactions of the Brown Pigments Re- 
actions of the Black Pigments. 

CHAPTER LXXI. EXAMINATION OF LAKES . . ... . . . 445 

Reactions of the Organic Colouring Matters. 

CHAPTER LXXII. THE TESTING OF DYE-WOODS 449 

The Colorimeter. 

CHAPTER LXXIII. THE DESIGN OF A COLOUR WORKS .... 457 
CHAPTER LXXIV. COMMERCIAL NAMES OF PIGMENTS .... 460 

APPENDIX. 

THE CONVERSION OF METRIC INTO ENGLISH WEIGHTS AND MEASURES 

CENTIGRADE AND FAHRENHEIT THERMOMETER SCALES . . . 469 

INDEX. 



UNIVERSITY 



CHAPTEE I. 

INTRODUCTION. 

IT is doubtful whether another branch of applied chemistry 
is recorded of so great an age as the colour industry ; at the 
present time there is hardly a race on the face of the earth 
which does not make use of colours in some form, either for 
the decoration of their persons or surroundings. The art of 
preparing colours is as ancient as their use. It is true that we 
find from the most remote historical records that the so-called 
earth colours were almost solely employed, and principally 
those which exist ready formed in nature. But these natural 
colours also require their particular process of preparation 
before they fulfil their object, even though this be merely a 
mechanical operation, such as powdering or levigating. 
That the oldest nations of whom we possess lasting records,, 
either written or otherwise, really understood the prepara- 
tion of colours by chemical processes is shown by the 
common occurrence in the Egyptian mural pictures of 
figures clad in brightly coloured garments, a proof that the 
Egyptians not only understood the science of colour manu- 
facturing, but also the more advanced art of fastening colours 
upon fabrics dyeing. 

The writings of the ancient Greeks, and in part also the 
scanty remains of their buildings, prove to us completely 
that they understood the use of colours to such an advanced 



53 MINERAL AND LAKE PIGMENTS. 

degree that they already employed them for pictures as works 
of art. That the Greeks were also acquainted with the 
preparation of colours and dyeing follows from various pas- 
sages from the classical writers, in which magnificently 
decorated rooms and beautifully coloured garments are often 
described. 

Among the Eomans, who were the pupils of the Greeks 
in the arts and manufactures, the prodigal luxury which 
existed in Borne, especially under the emperors, caused a 
great demand for colours, which were used in the most pro- 
fuse manner for the decoration of house and attire. The 
Roman colour makers had advanced so far in their art that 
they could colour the human hair rose red. 

A glance at East Indian fabrics and pictures, or at the 
ancient Chinese buildings, whose colouring is a matter of 
marvel to-day, shows that the Oriental were not behind the 
Western nations in the discovery of colours and the art of 
manufacturing them. 

In so old an industry it is not remarkable that great 
changes have taken place in the course of time. The 
thousands and thousands of experiments made by the al- 
chemists in the attempt to prepare gold failed in their main 
object, but the tremendous expenditure of time and trouble in 
this work was not fruitless ; upon the great mass 'of chemical 
facts discovered by the alchemists were laid the foundations 
of scientific chemistry. We find on reading the writings of 
the alchemists that the colour industry is indebted to them 
for an immense number of its products ; the reason being 
that the alchemists worked by preference on metals, earths 
and mineral compounds, and from these substances a large 
number of colours are obtainable, of which many are still in 
use to-day, and, on account of their cheapness, will continue 
in use. 

, The period in which the painters were also the colour 



INTRODUCTION. 3 

makers lies not far behind us. The preparation of many a 
colour of particular beauty was treated by the fortunate 
owner of the recipe as a great secret. It was sold by him 
at a great price. What a difference between that time and 
the present ! There is now no painter among civilised races 
to whom it would occur to prepare his own colours ; the 
chemical works provide them for him at a low price and in 
such a condition that they can be immediately used for 
painting. The Italian painters prepared the highly prized 
blue pigment, ultramarine, by laborious toil from the costly 
lapis lazuli ; to-day, this same colour, more beautiful and 
deeper in hue, is made by several works, and sold at a price 
which bears no comparison with that of the colour obtained 
from the mineral. The latter was worth many times its 
weight in gold : a pound of the finest ultramarine can now 
be bought for a shilling or two. 

We find a similar comparison in the case of the fine scarlet 
pigment known as vermilion : formerly the natural vermilion, 
cinnabar, was sold at a very high price ; at the present time 
the finest vermilion, prepared artificially, can be bought at 
a low rate. It is no longer necessary for any one to use 
natural Chinese vermilion as an artists' colour. 

Whilst formerly mineral colours were used in great 
preponderance, we now know a great number of vegetable 
and animal colouring matters. The discovery of the sea route 
to India and the discovery of America had an important in- 
fluence in this development. From these countries, as from 
other tropical lands, come the majority of the plants which 
contain colouring matters. The attempt to change these 
colouring matters into insoluble compounds led to the 
discovery of the lake pigments. 

With the advance of chemical knowledge the number of 
colours grew apace; e.g., the discovery of chromium was of 
great importance to the colour industry : it presented us with 



4 MINEEAL AND LAKE PIGMENTS. 

a large number of new colours. To a more limited extent, 
the discovery and study of uranium, molybdenum and other 
metals were the occasions for the invention of new colours. 

In more recent times, efforts in the colour industry have 
been especially directed to making colours more permanent 
and, at the same time, harmless. In the first respect, the 
position at present leaves much to be desired ; but, as regards 
the second property, great advances have been made. The 
colours in use in former days were almost all very poisonous 
compounds ; the greater number were derived from lead, 
copper, mercury or arsenic. More recently these poisonous 
substances have been in many cases replaced by innocuous 
materials, so that among the colours now in use, though the 
list is much more comprehensive than of old, there are but 
few poisonous to a high degree. 

In all civilised states the use of poisonous colours has 
been much restricted by law, and in those cases in 
which an article is to be manufactured for use as food 
the employment of such colouring matters has been abso- 
lutely forbidden. For example, in Germany by the law of 
5th July, 1887, concerning the use of dangerous colours in 
the preparation of foods and condiments, the application of 
the permissible colours has been exactly defined. 

During the last decades the colour industry and, still 
more, dyeing have undergone a complete change. The 
momentous discoveries which have been made in these 
departments leave far behind the advances which have been 

j_ 

made in other branches of chemical technology, the manu- 
facture of explosives, perhaps, excepted. We allude here to 
the beautiful colours which have been made from coal tar, 
colours which far surpass in beauty all hitherto known, and 
which we can already prepare in every shade and hue. Un- 
fortunately, we can only employ the coal-tar colours, as. 
such, in a restricted measure among the pigments ; they are 



INTEODUCTION. 5 

of more importance in dyeing. We use the term pigments 
here in the narrow sense of such substances which, when 
spread out on certain materials, provoke a certain sensation 
of colour. Dyeing is, on the contrary, that branch of colour 
chemistry which generally has for its object the simultane- 
ous production of the colour and its fixation upon a fabric. 
This definition was at least applicable to the majority of the 
colours which were in use before the discovery of the coal- 
tar colours and their introduction into the industry. Since, 
however, the latter have acquired so great a preponderance 
in dyeing, it is no, longer applicable, for the dyers use at 
present a large number of substances which are included 
in the narrow definition of pigments. The greater part of 
the coal-tar colours are substances which, in solution, when 
brought in contact with a fabric, adhere to it and colour 
it permanently. 

According to their use and preparation, pigments are 
divided into a number of classes, and one speaks of 
painters', artists', enamel, porcelain and glass colours, also 
of oil, honey, water and cake colours. Although this 
division is important for trade purposes, it is of little 
moment for the colour maker, for he can prepare the 
same colour for both purposes, either for oil or water- 
colour. What is of the greatest interest for the colour 
maker is the preparation of the pigment itself. The con- 
version of the prepared pigment into (oil or water) paint 
is unaccompanied by difficulties. 

When we look for a practical classification for pigments, 
we find that there are colours which exist ready formed in 
nature, arid others which can only be obtained by certain 
chemical processes, at times very complicated. 

As regards the first group of pigments those which 
exist ready formed in nature the processes which they 
undergo at the hands of the colour maker are almost en- 



6 MINERAL AND LAKE PIGMENTS. 

tirely mechanical treatments grinding, sieving, levigating 
and similar operations in order to convert them into such 
a condition that they can be used for painting. Since a 
large number of these pigments belong to that class of 
minerals which mineralogists call earths, these pigments 
have also been designated earth pigments, a term which we 
shall retain on account of its general use, although it is 
incorrect, since many of the so-called earth pigments are not 
obtained from " earths " in the mineralogical sense. 

Among the pigments which are prepared by human skill 
many divisions can be drawn. A large number of pigments 
are prepared from mineral sources ; an equally important 
number are derived from the animal and vegetable kingdom, 
the latter consisting of combinations of organic materials with 
certain inorganic substances. Some few pigments (putting 
aside the coal-tar colours) are simply organic products, as, 
for example, the majority of the black pigments, which 
consist of carbon. 

The following classification is drawn up on the lines in- 
dicated above : 

1. Natural Colours or Earth Pigments. Found ready formed 
in nature and requiring only mechanical preparation to be 
usable. A large number of handsome and also cheap colours 
belong to this class. 

2. Artificially Prepared Mineral Pigments. Obtained by cer- 
tain chemical processes, and, according to their composition, 
either compounds of metals with sulphur, oxygen, iodine, 
cyanogen, etc., or of oxides with acids, i.e., salts. 

3. Lakes. Compounds of colouring matters from the 
animal or vegetable kingdom with a mineral substance, such 
as lead oxide or alumina. 

As a fourth group we might take those colours which do 
not fall into the previous classes, as, for example, the black 
pigments composed of carbon ; but since this division is not 



INTEODTJCTION. 7 

made in practice we shall not regard this species of pigment 
as a particular group, but shall discuss them in the proper 
place. 

As an entirely new group of colours are to be classed 
those which are generally called coal-tar colours. These 
colours, which, at present, are the most important in dyeing 
and calico printing, are prepared from so-called organic 
compounds (more properly, carbon compounds). The manu- 
facture of these colours is a separate branch of chemical 
industry. 



CHAPTEE II. 

THE PHYSICO-CHEMICAL BEHAVIOUR OF PIGMENTS. 

IN a work which, as its title indicates, is devoted to a de- 
scription of the manufacture of pigments, the properties of 
those substances which are necessary for the preparation of 
colours cannot be exhaustively considered ; we must, there- 
fore, presuppose a knowledge of the elements of chemistry. 
We have to consider in this book the chemistry of colours ; 
the reader will, therefore, not expect an exposition of 
general chemical laws ; we shall only state certain facts 
which are of value to the manufacturer. With the descrip- 
tion of the manufacture of each pigment and of the materials 
required for that manufacture, we shall still discuss the 
chemical processes which must be conducted in the prepara- 
tion of the colours, so far as it is necessary in order to 
understand them. In this chapter we shall say a few words 
about the physical and chemical behaviour of pigments in 
general. 

The great majority of pigments are prepared by the pro- 
cess of precipitation, generally by mixing the solutions of 
two substances, upon which an interchange of the consti- 
tuents occurs and the less soluble compound separates in 
pulverulent form from the solution as a precipitate. Most of 
these colours are obtained by the admixture of the solutions 
of two salts ; the preparation of the so-called chrome yellow 
may be taken as an example. In the preparation of this 
pigment, a solution of a lead salt, sugar of lead (lead acetate), 



THE PHYSICO-CHEMICAL BEHAVIOUR OF PIGMENTS. 9 

is mixed with a solution of bichromate of potash, whereupon 
a precipitate of lead chromate (chrome yellow) is formed, 
whilst potassium acetate remains dissolved. The lead 
chromate is formed because the acetic acid has a greater 
affinity for potash than for lead oxide, wherefore an inter- 
change of acid and base takes place, but the lead chromate 
being insoluble in water consequently separates in the form 
of a precipitate. 

Many mineral pigments are produced in the form of pre- 
cipitates by passing sulphuretted hydrogen or carbonic acid 
gas into certain metal solutions. In these cases a similar 
exchange takes place between the reacting substances to 
that given in the case of chrome yellow ; the metals have a 
greater affinity for the sulphur or for the carbonic acid than 
for the substances with which they are already united, they 
unite with the former, and the new compound separates as 
an insoluble substance. We have examples of such com- 
pounds in cadmium sulphide, which is obtained by passing 
sulphuretted hydrogen into the solution of cadmium in an 
acid, and in white lead, which is formed by the saturation of 
a solution of lead acetate by carbonic acid. 

Many organic colouring matters, soluble in water, have 
the property of forming compounds with metallic oxides, 
soluble with great difficulty, when their solutions are mixed 
with a salt of lead, tin or aluminium, and the oxide is 
separated from the solution by an alkali. The precipitates 
obtained in this way are insoluble compounds of the colour- 
ing matter and the oxide of the metal ; they are called lake 
pigments, or, briefly, lakes. A large number of pigments, often 
of great beauty, is obtained in this manner. The lakes are 
widely used in all branches of painting and dyeing. 

Of great importance for the quality of the pigment is the 
physical condition of the precipitate ; this is either crystalline 
or amorphous, that is, non-crystalline. When a crystalline 



10 MINERAL AND LAKE PIGMENTS. 

precipitate is examined under the microscope, it is seen to 
consist of very small, coloured, transparent crystals. The 
amorphous precipitates are, however, in such a fine state 
of division that even with the highest magnification they 
transmit little or no light, and consequently appear opaque. 
These different characters of precipitates have the greatest 
influence on that property of pigments which we call covering 
power. In consequence of its transparency, a crystalline 
precipitate will allow the colour of the surface upon which 
it is spread to appear through, hence it must be laid on 
much more thickly than is necessary with an opaque pigment, 
of which a thin coating is sufficient to make the colour of 
the surface beneath invisible. 

How extremely important is the crystalline or non- 
crystalline nature of a precipitate in practice is seen by 
a consideration of white lead. This pigment, lead car- 
bonate, can be made by mixing solutions of a lead salt and 
a soluble carbonate (soda) ; but in this case a lead carbonate 
of crystalline nature is formed, which, being transparent, is 
of so small covering power that this process has no application 
in the manufacture of white lead ; but a far more troublesome 
method is used by which a non-crystalline product, amor- 
phous lead carbonate, is obtained. 

Many pigments are formed by burning (oxidising) metals, 
as, for example, zinc white ; others are prepared by melting 
salts together, as Naples yellow ; others again are formed by 
very complicated processes still partially unexplained, as is 
the case with ultramarine. In the manufacture of colours we 
find all chemical processes in use. 

It may be here remarked that it is quite possible to 
manufacture some colours, indeed a large number, accord- 
ing to fixed directions, without any particular chemical 
knowledge being necessary to carry on the processes. 
Indeed, in works we find most processes being carried 



THE PHYSICO-CHEMICAL BEHAVIOUE OF PIGMENTS. 11 

out by ordinary labourers who are quite destitute of any 
knowledge of chemistry. We must, however, add that we 
are convinced that any colour maker who works simply in 
a purely empirical manner, according to a stereotyped re- 
cipe, will never be in the position to raise himself above the 
position of a workman ; he will not be able, when a slight 
mishap occasions a change in the ordinary course of the 
process, to devise a means of overcoming the defect, but 
will be compelled to dispose of the faulty product in the 
condition in which it exists. Such a manufacturer is in a 
condition of blind dependence on the chemical works and 
dealers from whom he receives the raw materials requisite 
for the preparation of his colours. If he should receive 
materials which contain impurities not to be detected by 
empirical methods, the inevitable result will be that the 
colours produced from them will not be equal to the stan- 
dard. If, in making a colour which is the outcome of 
several processes, a workman once makes a mistake, the 
product will not be of the required quality. 

On the contrary, if the manufacturer possesses a certain 
amount of chemical knowledge, it will not be difficult for 
him to ascertain the causes of a failure in a process, and, at 
the same time, to devise means by which the defects may 
be removed. The manufacturer is more and more in the 
habit of buying the chemicals which he requires for his 
manufactures rather than of making them himself. He 
should, therefore, be in a position to form an opinion as 
to the usability and purity of these substances, which will 
only be possible when he has the knowledge requisite for 
subjecting them to a chemical examination. 

Although we shall presuppose, as we have said, that 
those who intend to concern themselves with colour manu- 
facturing possess an acquaintance with the principles of 
chemistry, yet this book has been so planned that it may 



12 MINERAL AND LAKE PIGMENTS. 

be of use (we hope) to the practical man who is innocent of 
chemical knowledge. On this account, we have devoted care 
to the description of those raw materials which are bought 
in large quantity, and to the simple investigation of their 
purity. 

When the manufacturer has the advantage of a chemical 
education, apart from his endeavours to produce colours 
lacking nothing in beauty or depth of shade, he will direct 
his endeavours in two directions, in respect of which great 
advances are yet to be made the permanence and harmless- 
ness of his colours. 

Many pigments possess the undesirable property of losing 
their brightness under atmospheric influences ; many, in- 
, deed, fade away completely in the course of time. We have 
only to examine a picture some centuries old ; in spite of the 
care bestowed on its preservation, we can say with certainty 
that, in the course of time, it will be so completely altered 
that nothing will remain of the original colours. It is the 
endeavour of the sensible manufacturer of colours to make 
only such as remain unaltered by atmospheric action, and 
also undergo no change when they are mixed with other 
pigments. Although it may be highly desirable that the 
painter should possess a knowledge of the chemical pro- 
perties of the colours he uses, still it should be the first 
object of the maker to take care that he places on the market 
only colours which will remain as much as possible unaltered 
when used alone, and will remain undecomposed when 
mixed. This is, unfortunately, not the case with many 
colours now in use. We shall return later to this point, 
of such extraordinary importance to the artist. 

The second point to be observed, is to produce only harm- 
less colours. The advances of chemistry have made known to 
us a series of colours which have the advantage over others 
known for a longer period that they are non-poisonous. 



THE PHYSICO-CHEMICAL BEHAVIOUR OF PIGMENTS. 18 

Unluckily, these harmless colours frequently fall behind the 
poisonous colours in brilliance, and generally they are more 
expensive. Here, too, is opened to the manufacturer a wide 
field of activity. The more completely poisonous substances 
disappear from the colours in use, the more widespread will 
be the use of colours. We should remark that the expres- 
sion " poisonous colours " is to be used with a certain 
reserve. Many pigments which contain lead, copper, anti- 
mony, mercury, etc., are poisonous, because they contain 
poisonous metals ; but poisoning with them will not readily 
take place on account of their insolubility. It is different 
with the very poisonous arsenic compounds, which should 
be removed from the list of colours in common use ; many a 
misfortune caused by them would then be avoided. 

Endeavours to. produce innocuous colours have been more 
successful than the efforts after permanence. There are now 
very few commonly used colours which can be accounted 
very poisonous compounds, and which cannot be replaced 
by other colours of equal beauty. On the whole, we are 
now in the position to prepare harmless colours suited to 
most purposes. Special endeavours should be made to sell 
these, so that such cases of poisoning should not occur as, 
for example, caused by gingerbread which had been wrapped 
in paper coloured by emerald green. 



CHAPTEK III. 

RAW MATERIALS EMPLOYED IN THE MANUFACTURE OF 
PIGMENTS. 

As we have mentioned before, the manufacturer of colours 
now generally uses materials supplied to him by chemical 
works. The purer these are, the easier it will be to work 
with them, and the finer will the colours turn out. We have 
indicated that it is important for the manufacturer to know 
accurately the properties of his materials in order to be able 
to estimate their value. Many substances required in cer- 
tain cases must be made by the colour manufacturer, since, 
on account of their condition, they cannot form articles of 
commerce chlorine and sulphuretted hydrogen, for example. 

In addition to the substances which are not to be bought, 
there are others which do occur in commerce, but are sold 
at so high a price that the manufacturer is compelled to 
make them himself. This is the case with the cobalt com- 
pounds, from which many beautiful colours are made. The 
producers of these demand such prices that it is to the 
interest of the colour maker to prepare them for his own 
use. 

In the following chapters, we shall deal with the more 
important raw materials which are employed in colour 
manufacturing, and shall restrict our remarks to what is of 
particular importance thereto. For more detailed accounts 
of these raw materials the reader is referred to the text 



RAW MATEEIALS USED IN MANUFACTURING PIGMENTS. 15 

books of chemistry, in which he will find them minutely 
described, in so far as they are chemical products. 

The materials employed may be divided into assistants in 
the processes and components of the manufactured pigment. 
The assisting substances are those which are used in the 
manufacture of a colour without entering into its composi- 
tion; from the component materials the colours are directly 
derived. For example, in the manufacture of Prussian blue, 
yellow prussiate of potash, an iron salt, water (in which the 
salts are dissolved) and nitric acid are used. In the blue 
obtained are contained portions of the iron salt and of the 
yellow prussiate, these are, therefore, component materials, 
whilst water and nitric acid are simply assistants, since they 
do not enter into the composition of the pigment. 

In colour making a large number of assisting materials 
are employed, which comprise a considerable number of 
elements and compounds. Since these are of great import- 
ance for our purpose, we shall describe their properties, and, 
when necessary, briefly the method of preparation. 

Among the component materials are to be reckoned a 
large number of salts of the alkaline earth and earth metals 
and of all the heavy metals. In addition, there are also 
the substances of animal or vegetable origin . used in lake 
making. 

In the description of the raw materials, if we were to 
overstep the line drawn here, we could include a great 
variety of compounds, those, for example, used in the 
^manufacture of the so-called aniline dyes. These substances 
form, however, as we have stated, the object of a particular 
branch of manufacture, which forms a separate division of 
colour chemistry, but with which is not to be confounded 
what has been hitherto designated the manufacture of 
colours. 



CHAPTEE IV. 

ASSISTANT MATERIALS. 

Water, H 2 = 18. 1 This substance plays a tremendous 
part in colour making ; almost all the substances which are 
used in solution are dissolved in water ; the removal from 
precipitates of admixed foreign bodies, the so-called washing, 
is always accomplished with water. The chemist does not 
understand by water quite that liquid which in general 
speech is so designated. We must consider the water which 
is at the disposal of the colour maker. 

Water, in the chemical meaning of the word, is a liquid 
composed only of hydrogen and oxygen, and leaving no 
residue when evaporated. Such water is not found in 
nature ; it can only be obtained by distillation of well or 
river water. The water which falls in long continued rain, 
or is obtained by melting snow, is most nearly like distilled 
water ; it contains only small quantities of dissolved sub- 
stances, and generally such as would be without influence 
in colour making. Water of this description is available for 
but a limited use ; the large quantities of water required in 
a colour works must be taken from springs or streams. 
These waters contain, however, more or less large amounts 
of dissolved salts, which act in a marked manner upon the 
substances dissolved in them. 

In almost all spring and well waters is found carbonate of 

1 We append the chemical formula and the molecular weight to the 
description of each compound. 



ASSISTANT MATEBIALS. 17 

lime ; such waters are called " hard ". Kiver water contains 
generally little carbonate of lime ; it is then called " soft 
water ". The influence of the carbonate of lime is especially 
evident when salts of lead, copper, iron and other heavy 
metals are dissolved in water ; the carbonate of the parti- 
cular metal gradually separates from solution, and the liquid 
becomes very turbid. 

When only hard waters containing much lime are at the 
service of the manufacturer, turbid solutions are often ob- 
tained, which must be filtered before use. In many cases 
this can be avoided by adding milk of lime to the water in 
a large vessel ; the free carbonic acid unites with the lime, 
and thus the carbonate of lime, which is only soluble in 
water containing free carbonic acid, separates as a fine pre- 
cipitate. Water which has been treated in this way be- 
comes clear after some time, through the deposition of the 
carbonate of lime ; it is then soft water. In order to separate 
the carbonate of lime in this way, no more than the requisite 
quantity of milk of lime should be added, so that no lime 
remains in excess, since this would cause precipitates when 
salts of lead, copper, iron, etc., were dissolved. In many 
cases for example, when lead or barium salts are dissolved 
the lime contained in the water can be made harmless by 
slightly acidifying with acetic or nitric acid. Water which 
contains sulphate of lime (gypsum) is equally useless for 
many purposes, as, for example, the solution of lead and 
Barium salts. These metals form insoluble compounds with 
the sulphuric acid, which render the solution turbid, and can 
be removed only with difficulty by filtering, on account of their 
great fineness. They are more easily removed by allowing 
to settle. 

Water containing gypsum often contains in addition 
small quantities of sulphuretted hydrogen. However small 
the quantities of this gas may be, they still make the water 

2 



18 MINERAL AND LAKE PIGMENTS. 

absolutely useless for certain purposes in the manufacture 
of colours ; for example, for the preparation of all pigments 
containing lead which are obtained by precipitation. The 
sulphuretted hydrogen forms black compounds with lead, 
copper, bismuth, mercury and other metals, which impair 
the brilliance of the colour. A colour made under these con- 
ditions is never clean, its hue is injured by the admixture of 
the black substance. 

Water which contains much common salt (sodium chlo- 
ride) is unsuitable for the solution of lead, mercury and silver 
salts. In consequence of the great affinity of these metals 
for chlorine, turbid solutions are obtained when their salts 
are dissolved in water containing common salt. 

Some waters contain a considerable quantity of iron. 
Such waters deposit on evaporation, and often on standing 
exposed to the air, a brown powder of ferric hydrate, which 
would have considerable influence on the shade of a pigment. 
White pigments, in the preparation of which such a water is 
used, have always a brownish tinge ; yellow and red pigments 
are also unfavourably affected. 

Carbonate of lime and common salt occur in small quan- 
tities in every well water. The colour maker must do the 
best he can with such a water ; its use will not particularly 
harm the shade of the colours prepared with it if the amount 
of the impurities is not very large. Water containing much 
iron is practically useless ; the oxide of iron would injure 
the colours so much that it would not be possible to obtain 
brilliant shades. Water from wells in the neighbourhood of 
deposits of turf or cemeteries often contains considerable 
quantities of organic substances which act injuriously on 
the shade of pigments ; such water should not be used in 
colour making. 

The impurities in a water are more or less harmful 
according to the purpose for which it is to be used. Bui- 



r. 



or TMI 
UNIVERSITY 



ASSISTANT MATERIALS. ^*^5i 19 

phate of lime is generally more injurious than carbonate of 
lime, since the precipitates which the latter causes in solu- 
tions of the salts of certain metals can be prevented by the 
addition of acids. This is not the case with sulphate of 
lime ; when lead or barium salts are dissolved in water 
containing this substance, a precipitate of lead or barium 
sulphate is obtained, which is insoluble. 

In dealing with the salts of costly metals, such as 
mercury or silver, it is better to dissolve them in distilled 
water, or, at least, very pure rain water. The rain water 
which runs from zinc or well tiled roofs is generally very 
pure ; for practical purposes it may be regarded as free from 
carbonate and sulphate of lime, sulphuretted hydrogen and 
common salt. The colour maker should take care to obtain 
as much of this pure water as possible by erecting large rain- 
water tanks. 

The less impurity a water contains the more useful it is 
for our purpose. After rain water soft river water is the 
best, and after this the softer well waters. All mineral 
waters distinguished by a high content of salts or gases are 
quite useless for colour making ; for this reason sea water is 
disqualified. 

An accurate analysis of a water is much too complicated 
for the manufacturer ; it is sufficient for him to convince 
himself of the absence of certain substances. Water which, 
some time after the addition of a little tannic acid solution, 
acquires a clear green or a bluish to black shade contains 
much iron, and is useless. Water which coagulates a large 
quantity of a solution of soap in alcohol is very rich in car- 
bonate or sulphate of lime. In order to decide approximately 
in what relative proportion these salts are present, a solution 
of barium chloride is added to the water so long as a pre- 
cipitate forms. If this disappears completely on the addition 
of nitric acid, the water contains only carbonate of lime ; if 



20 MINERAL AND LAKE PIGMENTS. 

it only partly dissolves, sulphate of lime is also present. 
The presence of chlorine is shown by a considerable tur- 
bidity on acidifying the water with nitric acid, boiling and 
adding silver nitrate. If the precipitate obtained on the 
addition of a lead salt is not pure white, but discoloured, 
the water contains sulphuretted hydrogen, which has formed 
black lead sulphide. In order to test the water for organic 
substances, about a litre is evaporated to dryness in a 
porcelain dish, and the residue heated to redness ; if it 
turns brown and black, and possibly gives off a smell of 
burnt feathers, the w r ater contains much organic matter. 

Pure water is coloured permanently red by a solution of 
potassium permanganate ; but if it contains organic matter, 
the solution is decolourised after some time and a brown 
precipitate is deposited at the bottom. From the amount 
of this precipitate an idea of the quantity of organic matter 
present may be obtained. 

It is only necessary to be very scrupulous concerning the 
quality of the water when it is to be used for the solution of 
salts or the extraction of dye-woods. For washing precipi- 
tates, which requires a large volume of water, there can 
generally be used, without detriment, water containing 
much lime, but it must be free from iron and sulphuretted 
hydrogen. The latter is particularly harmful to most of the 
lead colours, which would lose in beauty by washing with 
water containing this substance. 

It is hardly necessary to say that the water used in colour 
making must be quite clear. Muddy river water must in 
every case be completely freed from the solid particles con- 
tained in it, either by settling or by filtering. Filters filled 
with well-washed sand give good results for this purpose. 

Chlorine, Cl = 35'5. For some operations in colour mak- 
ing it is necessary to employ chlorine. This is a greenish 
yellow gas at ordinary temperatures, which is characterised 



ASSISTANT MATERIALS. 21 

by a suffocating smell and the energy with which it unites 
with most elements. On account of its injurious effects on 
man certain precautions have to be observed in preparing 
chlorine, and it is advisable to erect the apparatus necessary 
for its production in a separate room, so that the workmen 
are not injured by the gas. 

Formerly chlorine was exclusively made in lead apparatus, 
because this metal is one of the least readily attacked. When 
such an apparatus is used for the first time a layer of lead 
chloride is formed, which, like a varnish, protects the metal 
beneath from further attack. Fig. 1 represents an apparatus 




FIG. 1. 

formerly employed for the preparation of chlorine in chemical 
works. In the upper part of the pear-shaped vessel, K, there 
are four openings, two of which, D and C, are provided with 
water lutes. This means that the opening is surrounded by 
a moat containing water, into which the rim of the cover 
dips, thus making a joint. Through the middle opening goes 
the axle of the stirring apparatus, K ; in the fourth is a lead 
safety funnel, J. Solid materials are introduced through D, 
liquid through J ; the tube C carries away the chlorine 
formed; the tube A, furnished with a stop cock, can draw 
off the fluid contents of the apparatus. 



22 



MINERAL AND LAKE PIGMENTS. 



Since lead melts at low temperature, the apparatus can- 
not be heated over the fire without danger, therefore it is 
surrounded by an iron jacket, W, which is filled with water, 
or else the apparatus is heated by steam introduced into W. 
Larger quantities of chlorine are more conveniently pre- 
pared in an apparatus, of similar structure, made of stone or 




FIG. 2. 

earthenware, which have the advantage over lead that they 
are not at all attacked by chlorine. 

Fig. 2 exhibits the construction of such an apparatus of 
medium size. It is constructed of sandstone or earthen- 
ware ; the lid and some of the smaller parts can be made 
of either earthenware or lead. The pyrolusite is introduced 
at G in large pieces ; H is the funnel for pouring in the acid ; 



ASSISTANT MATERIALS. 23 

E K D, the steam pipe; C, the perforated false bottom 
upon which the pyrolusite lies ; F, the delivery tube for the 
chlorine ; /, the opening for running off the manganese 
chloride ; a, the leaden cover. 

To prepare chlorine, 1 part (by weight) of common salt, , 
1 part of powdered pyrolusite, 2J parts of vitriol and 1^ part 
of water are used. The salt and the pyrolusite are intro- 
duced through D into the apparatus (Fig. 1); the acid, 
diluted with the water, is poured in through the funnel, 
the materials are mixed by the stirrer and gently warmed 
until chlorine appears, when the application of heat must 
be considerably diminished or the chlorine will be violently 
evolved. 

Pyrolusite and hydrochloric acid are now generally used 
for the preparation of chlorine, because the solution of 
manganese chloride, left at the end of the operation, is 
valuable. 

If all the chlorine made in one operation is not at once 
required for the manufacture of a colour, it can be utilised 
by sending it into a box filled with slaked lime, which is 
converted into chloride of lime or bleaching powder. The 
liquid run away from the apparatus at the conclusion of the 
operation contains manganese and sodium sulphates, or 
manganese chloride as the case may be, and can be used for 
the preparation of manganese pigments. 

Ammonia, NH 3 =-- 17. Ammonia is obtained from 
chemical works in the form of a strong solution of am- 
monia gas in water, which is generally very pure. The 
density of an aqueous solution of ammonia is the smaller 
the more ammonia it contains, and thus the strength of a 
solution of ammonia can easily be formed by means of the 
hydrometer. The following table shows the percentage of 
ammonia, NH 3 , in a liquid of known specific gravity at the 
temperature of 14 C. : 



24 



MINERAL AND LAKE PIGMENTS. 



Specific Gravity. 


Ammonia per cent. 


Specific Gravity. 


Ammonia per cent. 


0-885 


36-00 


0-953 


11-50 


0-886 


35-00 


0-955 


11-00 


0-889 


34-00 


0-957 


10-50 


0-891 


33-00 


0-959 


10-00 


0-893 


32-50 


0-961 


9-50 


0-895 


32-00 


0-963 


9-00 


0-897 


31-30 


0-965 


8-50 


0-900 


26-00 


0-968 


8-00 


0-905 


25-39 


0-970 


7-50 


0-925 


19-54 


0-972 


7-00 


0-932 


17-52 


0-974 


6-50 


0-947 


13-46 


0-976 


6-00 


0-951 


12-00 


0-978 


5-50 



The Hydrometer. In the above table the percentage con- 
tent of the ammonia solution is given according to its 
specific gravity, that is, according to the ratio between the 
weight of any volume of the liquid and the weight of an 
equal volume of water. According to scientific principles, 
only those hydrometers should be used which are graduated 
in specific gravities. In spite of all exertions in this direc- 
tion, manufacturers have not yet been induced to use such 
instruments in every case. Hydrometers, with quite arbitrary 
scales, such as those of Baume and Twaddell, are frequently 
found in works. These hydrometers generally only show 
that a liquid is of so many degrees on the particular scale, 
and the manufacturer in using them is restricted to the 
following out of a certain recipe which requires the use of a 
liquid of a certain strength which is expressed in degrees 
Baume, etc. He does not learn by this how many per cent, 
of the particular substance are dissolved in the water when 
the liquid has a certain hydrometric strength. 

For the sake of uniformity, it is urgently to be desired that 
all manufacturers who use the hydrometer to estimate the 
content of a liquid in ammonia, potash, soda, hydrochloric, 
sulphuric, nitric acids, etc., should employ simple specific 
gravities. This is desirable, because the percentage strength 



ASSISTANT MATERIALS. 25 

of a solution, corresponding to the specific gravity, can be at 
once accurately found from tables. On these grounds, in the 
present work, we have restricted ourselves to tables showing 
simply the specific gravities of solutions and the correspond- 
ing composition. 

Sal Ammoniac or Ammonium Chloride, NH 4 C1 = 53'5. 
This substance comes into commerce in the form of a white 
crystalline meal, more rarely in the form of sugar loaves 
< crystallised sal ammoniac) or of flat cakes (sublimed sal 
ammoniac). It is usually very pure, since impure forms, 
generally containing much iron, are difficult of sale. At a 
particular temperature sal ammoniac is volatile ; it is used 
in certain mixtures in order to prevent the temperature, on 
heating, from rising beyond a certain point. Like ammonia, 
it is more used in dyeing. 

Ammonium Sulphide, NH 4 HS. This compound is ob- 
tained by leading sulphuretted hydrogen into ammonia 
solution so long as it is dissolved, and a test portion of 
the liquid still gives a white precipitate with a solution of 
magnesium sulphate. Ammonium sulphide decomposes by 
long standing in the air, sulphur being separated. It gives 
precipitates with the salts of certain metals, for example, 
iron, cobalt, manganese, zinc, nickel. These precipitates, 
'which consist of the sulphides of the metals, are not formed 
; by sulphuretted hydrogen in acid solutions. 

ACIDS. 

In colour making many acids are used for the solution 
of metals, the production of precipitates, for oxidations 
and so forth. Commercial acids, especially inorganic acids, 
generally contain not inconsiderable quantities of impurities 
which are injurious in the manufacture of many colours. 

Hydrochloric Acid, HC1 = 36'5. The commercial acid 
(muriatic acid, spirits of salt) generally contains large quan- 



MINERAL AND LAKE PIGMENTS. 



tities of iron, which colour it yellow fortunately, in many 
cases, this is not a disadvantage, and also at times the iron 
can be removed from solutions made in the acid. Another 
impurity is sulphuric acid. This can be detected by diluting 
and adding barium chloride ; if sulphuric acid be present, a 
white precipitate, or, at least, a cloudiness, appears. 

Ordinary hydrochloric acid is a solution of hydrochloric 
acid gas in water. The strongest acid contains 42'85 per 
cent, of the gas, and has the specific gravity 1'21. The 
following table gives the strengths of acids of various specific 
gravities : 



Specific Gravity. 


Hydrochloric Acid 
per cent. 


Specific Gravity. 


Hydrochloric Acid 
per cent. 


1-21 


42-85 


1-10 


20-20 


1-20 


40-80 


1-09 


18-75 


1-19 


38-88 


1-08 


16-71 


1-18 


36-36 - 


1-07 


15-49 


1-17 


34-34 


1-06 


13-86 


1-16 


32-32 


1-05 


11-49 


1-15 


30-30 


1-04 


8-97 


1-14 


28-28 


1-03 


6-93 


1-18 


26-26 


1-02 


4-89 


1-12 


24-24 


1-01 


2-03 


1-11 


22-22 









Sulphuretted Hydrogen, K,S = 34. This is a gas of 
acid properties smelling like rotten eggs ; it precipitates the 
sulphur compounds of many metals when led into the acicl 
solution of the corresponding salt. This substance is seldom 
required in colour works, so that it is convenient to have an 
apparatus which permits of the preparation of any required 
quantity. Fig. 3 represents an apparatus devised by the 
author, which is well adapted for the preparation of sul- 
phuretted hydrogen. It consists of a small, wooden tub, on 
whose upper edge lies a thick paper ring, so that the lid may 
be pressed down air-tight by the screws B. Through the lid 
pass a tap-funnel, T, a movable screw, S, and a tube, E, to 



ASSISTANT MATERIALS. 



27 



carry away the gas. On the screw S hangs a basket, K, by 
a handle ; this is filled with pieces of iron sulphide as large 
as nuts. The tub is filled to about one third of its height 
with a mixture of 9 parts of water and 1 part of sulphuric 
acid. 

When sulphuretted hydrogen is required the basket is 
lowered by the screw, S, until it dips in the liquid ; according 
as the basket dips more or less into the liquid a fast or slow 
current of the gas is obtained. When the gas is no longer 




required the basket is raised out of the liquid, and the evolu- 
tion of gas at once ceases. The funnel, T, serves for the 
introduction of the liquid, the tap, H, for drawing off the 
iron sulphate solution, which can be used with advantage 
for the preparation of fine iron colours. The apparatus 
should not be opened so long as sulphide of iron remains in 
the basket. 

Sulphuric Acid comes into the market in two different 
forms : oil of vitriol and fuming or Nordhausen sulphuric 



MINERAL AND LAKE PIGMENTS. 



acid ; both are used in colour making. Oil of Vitriol, H 2 S0 4 
= 98, is a colourless, oily liquid of high specific gravity ; it 
is generally tolerably pure, and contains, as a rule, only a 
small quantity of lead, the presence of which is indicated by 
a turbidity on largely diluting the acid. The amount of 
pure sulphuric acid in the liquid is practically determined by 
taking the specific gravity. The table indicates the relation 
between the specific gravity and the content of sulphuric 
acid. 



Specific Gravity. 


Sulphuric Acid 
per cent. 


Specific Gravity. 


Sulphuric Acid 
per cent. 


1-8485 


100 


1-8043 


89 


1-8475 


99 


1-7962 


88 


1-8460 


98 


1-7870 


87 


1-8439 


97 


1-7774 


86 


1-8410 


96 


1-7673 


85 


1-8376 


95 


1-7570 


84 


1-8336 


94 


1-7465 


83 


1-8290 


93 


1-7360 


82 


1-8233 


92 


1-7245 


81 


1-8179 


91 


1-7120 


80 


1-8115 


90 









Nordliausen Sulphuric Acid, H 9 S 9 T = H.,S0 4 + S0 a , is gener- 
ally a yellowish brown liquid, which gives off white fumes in 
the air. It contains varying quantities of sulphur trioxide 
-dissolved in sulphuric acid. It often contains selenium, 
which separates as a red powder when the acid is diluted. 
The presence of this impurity does not interfere with the use 
of the acid for dissolving indigo, the only purpose for which 
it is required in the colour factory. 

Nitric Acid, HN0 3 = 63. This acid, which is used in 
the preparation of many colours, is distinguished by the 
readiness with which it gives up part of its oxygen, and thus 
converts metals like antimony and bismuth into oxides, and 
transforms other compounds into a higher state of oxidation. 
There are two kinds of nitric acid : ordinary nitric acid, a 



ASSISTANT MATERIALS. 



colourless liquid which is more or less pure ; and fuming nitric 
acid, a yellow or orange coloured liquid, fuming strongly in 
the air, which consists of a solution of nitrogen peroxide,. 
N0. 2 , and nitric oxide, NO, in nitric acid. 



Specific Gravity 


Nitric Acid per 


Specific Gravity 


Nitric Acid per 


at 15 C. 


cent. 


at 15 C. 


cent. 


1-530 


100-00 


1-323 


55-00 


1-520 


97-00 


1-284 


50-49 


1-509 


94-00 


1-251 


45-00 


1-503 


92-00 


1-211 


40-00 


1-499 


91-00 


1-185 


33-86 


1-495 


90-00 


1-157 


30-00 


1-478 


85-00 


1-138 


25-71 


1-460 


80-00 


1-120 


23-00 


1-442 


75-00 


1-089 


20-00 


1-423 


69-96 


1-067 


15-00 


1-400 


65-07 


1-022 


11-41 


1-346 


60-00 


1-010 


4-00 



Since the action of nitric acid chiefly depends on its 
oxidising properties, which are possessed by both kinds, it 
generally does not matter which is used. The usual im- 
purities are chlorine and sulphuric acid ; the presence of the 
first is shown by silver nitrate solution, of the latter by 
barium chloride, in each case added after diluting. When 
the acid is used for oxidations these impurities do not inter- 
fere, but nitric acid containing chlorine cannot be used to 
dissolve silver, because the chlorine would form insoluble 
silver chloride. 

The strength of nitric acid is gauged by its specific 
gravity as given in the table. 

Aqua Regia, A mixture of 2 parts of hydrochloric acid 
and 1 part of nitric acid gradually turns orange or yellow 
and evolves chlorine. This liquid, which can dissolve 
gold in consequence of the free chlorine it contains (hence 
its alchemistic name, from gold, the "king of metals "), is 
used as a very powerful oxidising agent in colour making. 



"30 MINERAL AND LAKE PIGMENTS. 

Carbon, C = 12, is the only one of the lion-metallic 
elements to be mentioned here ; by itself it forms a group 
of very important pigments, which we shall describe in 
detail at a later stage. 

Carbonic Acid Gas, C0 2 = 44, is used in the manufacture 
of white lead, which it precipitates from lead acetate. This 
is, however, a particular branch of colour making carried on 
in special works. In describing this manufacture we shall 
return to the preparation of carbonic acid on a large scale. 



ORGANIC ACIDS. 

The organic acids which are important in colour making 
are acetic, oxalic and tartaric acids. 

Acetic Acid, C,H 4 2 = 58. The very dilute form of this 
substance is known commonly as vinegar, the stronger as 
pyroligneous acid, and the purest as glacial acetic acid ; the 
latter is, however, scarcely used. Formerly in colour mak- 
ing ordinary vinegar was used, but now pyroligneous acid is 
almost exclusively employed. This is distinguished by its 
strong empyreumatic smell, which, however, is without 
importance in colour making. 

The strength of a solution of acetic acid cannot be found 
by a simple estimation of specific gravity, since the density, 
does not increase with the percentage of acetic acid. If an 
accurate estimation of the strength of acetic acid is required, 
it must be obtained by neutralising the acid with an alkali 
by a process of volumetric analysis. 

For practical purposes, where it is generally known 
whether a very strong or a more dilute acetic acid is under 
consideration, the following table, showing the connection 
between specific gravity and percentage strength, is suf- 
ficient. 



ASSISTANT MATERIALS. 



31 



Specific 
Gravity. 


Acetic 
Acid 
per cent. 


Specific 
Gravity. 


Acetic 
Acid 
per cent. 


Specific 
Gravity. 


Acetic 
Acid 
per cent. 


Specific 
Gravity. 


Acetic 
Acid 
per cent. 


1-0635 


100 


1-0720 


74 


1-058 


48 


1-031 


22 


1-0670 


98 


1-0710 


72 


1-055 


46 


1-027 


20 


1-0690 


96 


1-0700 


70 


1-054 


44 


1-025 18 


1-0706 


94 


1-0700 


68 


1-052 


42 


1-023 


16 


1-0716 


92 


1-0690 


66 


1-051 


40 


1-020 


14 


1-0730 


90 


1-0680 


64 


1-049 


38 


1-017 


12 


1-0730 


88 


1-0670 


62 1-047 


36 


1-015 


10 


1-0730 


86 


1-0670 


60 


1-045 


34 


1-012 


8 


1-0730 


84 


1-0660 


58 


1-042 


32 


1-008 


6 


1-0730 


82 


1-0640 


56 


1-040 


30 


1-005 


4 


1-0735 


80 


1-0630 


54 


1-038 


28 


1-002 


2 


1-0732 


78 


1-0620 


52 ! 1-035 


26 


1-001 


1 


1-0730 


76 


1-0600 


50 


1-033 


24 









Oxalic Acid, C 2 H,0 4 .2H 2 = 126, has but a limited use 
in colour making. It comes into commerce in the form of 
more or less pure white crystals which readily dissolve in 
water, and are almost pure oxalic acid, containing only 
small quantities of oxalate of lime, the presence of which 
is without importance for the purposes to which the acid is 
put in colour making. Frequently, instead of oxalic acid, 
the acid potassium oxalate (salt of sorrel) is used. 

Tartaric Acid, C 4 H G 6 = 150, occurs as white or yellowish 
crystals, with a slightly burnt smell, which dissolve readily 
in water, and have a strong acid taste. The pure acid, 
which is white and without smell, is considerably dearer 
than the yellow variety. The impurities of the latter, 
which are small in quantity, are without influence on the 
colours prepared by its help, so that this form is generally 
-used. 



CHAPTER V. 

METALLIC COMPOUNDS. 

ALKALIS. 

THE compounds of the alkali metals, potassium and sodium,, 
play a considerable part in colour making. Formerly 
the potassium compounds were in general use, but the 
sodium compounds are at present obtainable at a much 
lower price, and in most cases they can be used equally 
well. Thus, in colour making, sodium compounds are 
chiefly employed. The cyanogen compounds are an ex- 
ception ; their potassium compounds are used exclusively. 

Potassium Compounds. The potassium compounds which 
are chiefly used in colour making are potassium carbonate 
(potashes, pearl-ash), potassium hydroxide (caustic potash), 
potassium nitrate (saltpetre), potassium tartrate (tartar), and 
potassium ferrocyanide and ferricyanide (yellow and red 
prussiate of potash). The cyanogen compounds have pecu- 
liar properties. We shall describe them separately after the 
potassium and sodium compounds. 

Potassium Carbonate (carbonate of potash), K 2 C0 3 = 138 r 
is known commercially as potashes, a name derived from its 
former method of preparation by heating the ashes of plants 
in pots. At present potashes are prepared in large quantities 
from other sources. 

Pure potash forms crumbling lumps with a slight yellow 
or bluish grey tinge, rapidly absorbing moisture from the 
air, and in time completely liquefying. The yellowish tinge 



METALLIC COMPOUNDS. 33 

is caused by oxide of iron, the bluish by manganese com- 
pounds. The so-called calcined potash has been strongly 
heated, and thus all organic substances contained in it have 
been destroyed. 

Potashes are in no way pure potassium carbonate ; they 
contain a mixture of all those salts which are found in 
plants potassium sulphate and chloride, small quantities of 
silicic acid, etc. These impurities are rarely harmful, still 
it is generally necessary to know the percentage of pure 
potassium carbonate contained in potashes. 

Although at present in commerce the strength of potashes 
is frequently guaranteed, it is still desira.ble to estimate the 
strength. It is sufficient for practice to allow a small quan- 
tity, say 100 grammes, to stand with an equal quantity of 
very cold water for some hours, then to filter and pour a 
similar quantity of water over the residue on the filter. 
The weight of undissolved substance subtracted from 100 
gives with sufficient accuracy the weight of pure potassium 
carbonate contained in 100 parts of potashes. This method 
is founded on the fact that potassium carbonate dissolves 
readily even in cold water, but the other salts with difficulty. 
This procedure can also be used to obtain pure potassium 
carbonate from crude potashes ; it is only necessary to filter 
and evaporate to dryness the solution obtained by pouring 
very cold water on crude potashes. 

Potassium Hydroxide (Potassium Hydrate, Caustic Potash), 
KOH = 56. The commercial variety consists of very de- 
liquescent white lumps, generally containing a large quantity 
of impurities. On this account caustic potash, or rather a 
solution of it, is prepared in the colour works. 

With this object 11 parts of potash, contained in a tub 
with an opening at the bottom, are mixed with 100 parts of 
cold water. Two hours later, the clear solution is run off 
into a clean iron pan, in which it is heated to boiling. To 



34 



MINEEAL AND LAKE PIGMENTS. 



the boiling solution is added milk of lime prepared from 
water and 3*5 parts of quicklime. After the liquid has 
boiled a few minutes, a small portion is filtered and hydro- 
chloric acid added to the clear filtrate ; if no effervescence 
occurs, then all the potassium carbonate is converted into 
caustic potash. Should effervescence occur, milk of lime is 
added until a new portion no longer effervesces on the addi- 
tion of hydrochloric acid. Then the pan is covered with a 
well-fitting lid, and the cooled liquid, if not required for 
immediate use, preserved in well-corked glass bottles. 

The strength of a caustic potash solution can be found 
by means of a hydrometer. The following table shows the 
relation between the specific gravity of a solution and the 
percentage of caustic potash it contains : 



Specific Gravity. 


Caustic Potash per cent. 


Specific Gravity. 


Caustic Potash per cent. 


1-06 


4-7 


1-28 


23-4 


1-11 


9-5 


1-39 


32-4 


1-15 


13-0 


1-52 


42-9 


1-19 


16-2 


1-60 


46-7 


1-23 


19-5 


1-68 


51-2 



Potassium Nitrate (Saltpetre), KNO 3 = 101, consists of 
large crystals, which quickly dissolve in water. On heating 
it readily gives up oxygen, and thus finds use as an oxidising 
agent. In former times, when the colour manufacturer was 
compelled to make his own materials, saltpetre was of great 
importance in colour making ; at present, when such mate- 
rials are to be bought at low prices and no colour maker 
prepares his own, saltpetre is little used. 

Potassium Bitartrate, C 4 H,,KO, ; = 188. This salt, known 
as tartar in large crystals, and as cream of tartar in the form 
of meal, is occasionally used in colour making. It is little 
soluble in cold water, but more easily in hot. The hot 
solution is generally used. 



METALLIC COMPOUNDS. 35 

Potassium Bichromate (Bichromate of Potash), K 2 Cr 2 7 
= 295. This salt is made in special works, by melting 
chrome iron ore with saltpetre and extracting the mass with 
water, when a yellow solution of potassium chromate is 
obtained ; to this sulphuric acid is added, which unites 
with half the potassium, thus leaving potassium bichromate, 
which is obtained by evaporation of the solution in fine red 
crystals. These are purified by recrystallisation. At present, 
in place of the above method, calcium chromate is formed by 
roasting chrome iron ore with lime ; the calcium chromate 
is then decomposed by a soluble potassium salt, thus form- 
ing potassium chromate. 

Potassium bichromate is unaltered in air ; it dissolves 
easily in water, and is of great importance in the prepara- 
tion of many colours, in particular chromium oxide and 
the lead pigments. The commercial salt generally contains 
potassium sulphate, with which at times it is intentionally 
adulterated. The adulteration is detected by dissolving in 
water, adding half the volume of pure hydrochloric acid, and 
cautiously and carefully dropping in spirits of wine. A rapid 
action takes place, which is only assisted by warming when 
necessary. The red liquid changes to emerald green. If 
barium chloride be now added, a white precipitate is ob- 
tained in the presence of potassium sulphate. 

Potassium Sodium Chromate, KNaCr0 4 = 279, is also 
used in colour making. Its solution is made by adding soda 
to a solution of potassium bichromate so long as an efferves- 
cence of carbonic acid occurs, and until the liquid turns 
red litmus paper blue ; the solution of the double salt is 
yellow. 

Chrome Alum, KCr(S0 4 ) 2 .12H 2 0= 499. This salt occurs 
in commerce as beautiful violet crystals. It is obtained as 
a by-product in the manufacture of aniline and anthracene 
dyes, and may often be bought at lower prices than other 



36 MINEEAL AND LAKE PIGMENTS. 

chromium salts; 100 parts of water dissolve approximately 
20 parts of chrome alum. 

Potassium Ferrocyanide, K 4 Fe(CN) 6 . 3H,0 = 422. The 
potassium iron cyanogen compounds are made in special 
works, particularly in the neighbourhood of large towns, by 
melting potashes with nitrogenous organic substances and 
iron, washing out the mass and purifying the salt so obtained 
by recrystallisation. Potassium ferrocyanide (yellow prus- 
siate of potash) forms large transparent crystals of a peculiar 
soft nature, which dissolve readily in water. It often con- 
tains considerable quantities of potassium sulphate, up to 
5 per cent., and it is to be noted that the impurity is much 
the cheaper of the two salts. When barium chloride is 
added to a solution of the salt, a white precipitate forms 
if sulphate be present. 

The behaviour of yellow prussiate towards iron salts is 
noteworthy. With ferrous salts, for example green vitriol 
(copperas), it gives a white precipitate which gradually turns 
blue in the air ; with ferric salts, for example ferric chloride 
(" nitrate of iron "), it at once gives a blue precipitate. 

Potassium Ferricyanide (Ked Prussiate of Potash), 
K 3 Fe(CN) 6 329, is obtained by passing chlorine through 
a solution of yellow prussiate until the liquid smells strongly 
of chlorine and no longer gives a precipitate with a solution 
of a ferric salt. The solution then contains potassium ferri- 
cyanide and chloride. The former is obtained by evaporat- 
ing and allowing to crystallise. 

Pure potassium ferricyanide forms beautiful dark red 
crystals, which readily dissolve in water. The solution 
gives a blue precipitate with ferrous salts, but only a brown 
colouration and no precipitate with ferric salts. Both yellow 
and red prussiate are used in the preparation of several 
much-used colours, for Prussian and Chinese blues, and 
several others. All cyanogen compounds, with the excep- 



METALLIC COMPOUNDS. 



37 



tion of yellow prussiate, are extremely poisonous. The 
following table gives the solubility of potassium ferricyanide 
at different temperatures : 



100 Parts of Water 
dissolve Parts of Salt. 


Temperature. 


Specific Gravity 
of Solution. 


00. 


33 
36 


4-44 

10-00 


1-151 
1-164 


40-8 


15-50 


1-178 


58-8 


37-80 


1-225 


77-5 


100-00 


1-250 


82-6 


104-40 


1-265 



Sodium Salts, In chemical properties the sodium salts 
are very similar to the potassium salts, and, being cheaper, 
they are generally used in place of the latter. 

Sodium Carbonate (Soda Crystals), Na,C0 3 .10H 2 = 286, 
is made in enormous quantities in great works and in a 
very pure state. It forms large transparent crystals, which 
effloresce in the air, losing a large quantity of water, and so 
falling to a white powder. Although this property does not 
interfere with the use of soda, since it is generally used in 
solution, yet efflorescence should be as far as possible avoided 
by keeping the salt in well-closed packages, because efflor- 
esced soda dissolves more slowly than crystallised, since it 
has to combine with water before it can enter into solution. 

In the retail trade a form of soda is found which is adul- 
terated with very large quantities of Glauber's salt. This is 
recognised by the different form of the crystals. Manufac- 
turers sell soda stating its strength. The colour maker 
should only buy with this guarantee. 

Sodium Hydroxide (Sodium Hydrate, Caustic Soda), 
NaOH = 40, comes into commerce in the form of hard 
masses, the use of which would be very convenient for 
the colour maker if it were not often very impure. Thus 



38 



MINEEAL AND LAKE PIGMENTS. 



it is better to prepare a solution oneself, which is accom- 
plished in the manner given above for caustic potash. 

Caustic soda and caustic potash have similar properties ; 
they have a corrosive action on the skin, readily unite with 
carbonic acid from the air, and separate the heavy metals 
from their solutions in the form of hydrated oxides. 

The strength of caustic soda solutions is given in the 
following table : 



Specific 
Gravity. 


Caustic 
Soda per 
cent. 


Specific 
Gravity. 


Caustic 
Soda per 
cent. 


Specific 
Gravity. 


Caustic 
Soda pei- 
cent. 


Specific 
Gravity. 


Caustic 
Soda per 
cent. 


2-00 


77-8 


1-56 


41-2 


1-40 


29-0 


1-23 


16-0 


1-85 


63-6 


1-50 36-8 


1-36 


26-0 


1-18 


13-0 


1-72 


53-8 


1-47 34-0 


1-32 


23-0 


1-12 


9-0 


1-63 


46-6 


1-44 31-0 


1-29 


19-0 


1-06 


4-7 



Besides soda and caustic soda, few soda salts are used in 
colour making. However, sodium nitrate (Chili saltpetre), 
NaNO 3 , is frequently used instead of ordinary saltpetre, from 
which it differs in being deliquescent. 

Sodium Thiosulphate (Hyposulphite), Na 2 S 2 3 .5H 2 = 
248, is a common article of commerce, being much used by 
photographers ; it is used in a few cases in colour making. It 
forms large crystals with a somewhat bitter taste, permanent 
in air and readily soluble in water. 

Sodium Chloride (Common Salt), NaCl = 58'5, which has 
a little application in colour making, is sufficiently pure 
in the form in which it is generally used for household 
purposes. 

SALTS OF THE ALKALINE EARTH METALS. 

The metals which are known as the alkali earth metals 
have much similarity with the alkali metals in their com- 
pounds, with the difference that their alkalinity is much 
less, and that their salts are much less soluble in water. For 



METALLIC COMPOUNDS. 39 

colour making the compounds of three of these metals 
calcium, barium and magnesium are used. 

CALCIUM COMPOUNDS. 

The most important calcium compounds are lime and 
carbonate and phosphate of lime. Carbonate of lime is used 
as a pigment, and will be dealt with in detail among the 
mineral colours; it will be but briefly described here. 

Calcium Oxide (Quicklime), CaO = 56. When chalk and 
limestone, which consist of calcium carbonate, are heated, 
carbonic acid is evolved, and calcium oxide, commonly 
called quicklime, is left. For our purposes only very pure 
quicklime is to be used. Its ordinary impurities are iron 
oxide and magnesia : the former is found in lime made from 
red or brown limestone ; the latter in lime made from dolo- 
mitic limestone. The presence of oxide of iron is recognised 
by the reddish tinge of the quicklime ; if magnesia be present, 
a small quantity of the quicklime, when mixed with a very 
large quantity of water, leaves an insoluble residue which 
consists of magnesia. 

When quicklime and water are brought together they 
unite very energetically and form calcium hydroxide or 
slaked lime. According to the quantity of water used for 
slaking, either dry slaked lime, lime paste, or milk of lime 
is produced, all of which find a use in colour making. 

Calcium Hydroxide (Slaked Lime), Ca(OH).j - 74. In 
order to prepare slaked lime, which contains lime united 
with just the necessary quantity of water, the pieces of 
quicklime are sprinkled with water from a watering can. 
The water is rapidly taken up and the sprinkling is repeated 
until the lumps begin to fall to a fine powder ; in the process 
the lime becomes very hot. The slaked lime is then passed 
through a sieve in order to separate the larger pieces of 
quicklime which have not been slaked ; the powder must 



40 MINERAL AND LAKE PIGMENTS. 

be kept in well closed packages, since it energetically absorbs 
carbonic acid out of the air. 

If so much water is added to the lime that a homogeneous 
wet mass is formed which can be readily moved with a 
shovel, one has then lime paste, which can be conveniently 
kept in pits as the masons do ; it may be stored in this way 
'for many months without appreciable alteration, still it is 
better to keep it covered. To prepare milk of lime, so much 
water is used in slaking that a milky liquid is formed, or 
the lime paste is mixed up in the proper quantity of water. 
Slaked lime dissolves in 700 to 800 parts of water ; on stand- 
ing, the undissolved slaked lime settles to the bottom of the 
milk of lime : thus it is better to prepare milk of lime im- 
mediately before use, and to stir it well to prevent the 
settling of the solid particles. 

Slaked lime in one of its forms is often used instead of 
the more costly caustic soda in order to precipitate metallic 
oxides from their salts. 

At times one finds a too strongly burnt lime, so-called 
" dead-burnt " lime, which is very slowly slaked by water. 
Such quicklime is slaked by allowing it to lie in water for 
days, or by means of hot water, which accomplishes the 
slaking more quickly. 

Calcium Carbonate, CaC0 3 = 100, is found naturally in 
large quantities as chalk, which consists of the skeletons of 
extremely small animals. By powdering and levigating, it 
is converted into a soft powder, which is used to lighten the 
shade of lakes and other colours. 

Calcium Sulphate (Gypsum), CaSO 4 .2H,0 = 172. - 
This mineral, when finely powdered, is added to some 
colours. 

Calcium Phosphate (Bone Ash), Ca 3 (P0 4 ). 2 = 810, is some- 
times used to lighten the shade of certain colours which 
might be injured by calcium carbonate. It comes in large 



METALLIC COMPOUNDS. 41 

quantities as a fine, white powder from South America. 
Naturally, only quite white bone ash can be used ; if not 
completely burnt it is grey, and will then impair the shade 
of colours with which it is mixed. 

Magnesium Carbonate (Magnesia), MgC0 3 = 84, is also 
used as an addition to colours in order to obtain pale shades. 
It is most cheaply obtained by dissolving magnesium sul- 
phate (Epsom salts") in water and adding soda solution so 
long as a precipitate forms, which is then washed and dried. 
The magnesium carbonate prepared in this way is a very fine, 
light powder, insoluble in water, which can be mixed with the 
most delicate colours without harming them. White magnesia 
is an extremely light powder, which may be used when it can 
be bought at as low a price as the above preparation. 

Barium Compounds. The raw material used for the pre- 
paration of barium pigments is either barium sulphate (barytes, 
heavy spar) or barium carbonate (witherite). The latter is 
much more rare than barytes, which is almost exclusively 
employed in the preparation of barium compounds. The 
barium compounds of particular importance for our purpose 
are barium chloride and nitrate. 

Barium Chloride, BaCL . 2H,0 = 244. -This salt is now 
a common article of trade, and can be bought at a low price. 
When pure it forms colourless crystals readily soluble in 
water. If the colour maker is able to get cheap barytes and 
fuel, it may be advantageous for him to prepare barium 
chloride himself. 

To prepare barium chloride from barytes, the latter is 
very finely ground and levigated, intimately mixed with coal, 
and the mixture subjected to a very high temperature, when 
barium sulphide is formed, which is dissolved by washing 
out the mass with water and converted into barium chloride 
by adding hydrochloric acid, sulphuretted hydrogen being 
evolved. 



42 MINEEAL AND LAKE PIGMENTS. 

The best method is to mix 4 parts of barytes with 1 
part of bituminous coal and so much coal tar that a plastic 
mass is formed, which is well kneaded and made into small 
cylinders 3 centimetres in diameter and 10 centimetres long. 
These cylinders are placed in layers in a cylindrical furnace 
with a good draught, which contains at the bottom a layer of 
coal 15 to 20 centimetres thick, then a layer of the cylinders^ 
then again coal, and so on until the furnace is full. The lowest 
layer of coal is lighted, and the whole burnt at a bright red 
heat, when the barium sulphate is changed into sulphide. 
Hydrochloric acid is poured over the residue and the insoluble 
part, consisting chiefly of unaltered barytes, is used for the 
next operation. 

Witherite (barium carbonate) can be converted into 
barium chloride in a very simple manner. Hydrochloric 
acid is added, in which it dissolves with the evolution of 
carbonic acid. The solution is allowed to stand twenty-four 
hours with excess of witherite ; the whole of the dissolved iron 
is thus precipitated. The solution is then filtered, evaporated 
down and left, when pure barium chloride crystallises out. 

Barium chloride and all soluble barium salts should only 
be dissolved in pure water (rain or distilled). Water which 
contains carbonates or sulphates always gives a turbid solu- 
tion by precipitating barium carbonate or sulphate. 

ALUMINIUM COMPOUNDS. 

The compounds of the earth metal aluminium play a 
very important part in colour making, since they form 
beautifully coloured compounds with many organic colour- 
ing matters. Formerly alum was the only material used 
in colour factories for the preparation of the alumina com- 
pounds ; at present aluminium sulphate is used, and when 
it is sufficiently pure it is the most valuable material,, 
because it contains the greatest quantity of alumina. 



METALLIC COMPOUNDS. 43 

Under the designation of alum only one compound, the 
so-called potash alum, was at one time found in commerce, 
but now there are other alums, which contain soda or 
ammonia in place of potash. These salts are of equal use 
in colour making to potash alum. The preference is to be 
given to the compound which contains the largest proportion 
of alumina. The chief point to be observed in connection 
with alumina compounds for use in colour making is that 
they shall be free from iron, because iron oxide, which would 
be precipitated out of the solution along with the colours, in 
consequence of its red colour would spoil the shade of the 
pigment. 

Aluminium Sulphate (Sulphate of Alumina), AL,(SO 4 ) 3 . 
18H 2 = 664. Any manufacturer who can obtain cheap 
china clay (kaolin) and sulphuric acid can himself prepare this 
compound with advantage. The apparatus used for this pur- 
pose is an iron pan containing sand, in which is placed a large 
earthenware dish. In this dish are put very finely-ground 
kaolin and strong sulphuric acid, and the mixture is heated 
so strongly that the acid boils, evolving heavy, white vapours. 
It is absolutely necessary to heat in this manner in order to 
avoid dangerous accidents. Sulphuric acid bumps so violently 
on boiling that it may even break a thick earthenware dish. 
The use of a sand bath makes the bumping harmless. 

China clay, which consists of silicate of alumina, is 
decomposed by heating with sulphuric acid into silicic acid 
and sulphate of alumina. The original milky liquid becomes 
more transparent during boiling, and has at last the appear- 
ance of starch paste. Kaolin contains varying quantities of 
silica. The quantity of sulphuric acid necessary for its de- 
composition can only be found by trial. The quantities are 
chosen so that a small amount of kaolin remains undecom- 
posed in order that the aluminium sulphate shall contain no 
free sulphuric acid. 



44 MINERAL AND LAKE PIGMENTS. 

When the decomposition is finished the pan is allowed 
to cool and the solid mass is brought into a vat filled with 
water, in which it is stirred until dissolved ; then the liquid 
is left until the jelly-like mass of silicic acid has sunk to the 
bottom, when the clear solution of aluminium sulphate is 
drawn off and can at once be used. 

If solid aluminium sulphate is required and this is to 
be recommended when large quantities are to be prepared 
the solution is evaporated in earthenware dishes until a 
portion solidifies when dropped on a cold plate. The molten 
aluminium sulphate is then cast in prismatic blocks, which 
are preserved in boxes. These blocks are of a pure white 
colour and very crystalline ; they dissolve with difficulty in 
cold, but readily in hot water without residue. The solution 
has an acid taste, even when it contains no excess of sul- 
phuric acid. When the blocks have a yellowish tinge, this 
denotes the presence of iron, and the solution must be freed 
from this impurity. Nowadays sulphate of alumina can be 
obtained so cheaply that it is hardly of advantage to make it. 

THE ALUMS. 

These are double salts of aluminium sulphate and 
potassium, sodium or ammonium sulphate. There are also 
other double sulphates known as alums which, in place of 
aluminium, contain chromium, iron, etc., but they are not 
of interest here. It may still be observed that all alums, 
whatever their composition, possess the property of crystal- 
lising together from mixed solutions, so that crystals can be 
obtained in which every existing alum is contained. 

The potassium, sodium and ammonium aluminium alums 
are used in colour making. 

Potassium Aluminium Alum, KA1(S0 4 ). 2 .12H 2 O = 474. 
This is the substance commonly called alum. Like all alums 
it crystallises in fine octahedral crystals, which at first are 



METALLIC COMPOUNDS. 



quite transparent, but slowly effloresce in the air and be- 
come covered by a white powder. It dissolves with difficulty 
in cold, but readily in hot water. The solution has at first 
a sweet taste, with an astringent after-taste. 

Alum comes into the market in different forms, of which 
the following are the most important : as crystallised alum, 
in the form of large crystals united together ; as alum meal, 
a coarse crystalline powder obtained by rapidly cooling and 
stirring a hot alum solution. On account of the larger 
surface this form dissolves more quickly than the large 
crystals. Roman alum is the name of a variety chiefly 
imported from Italy ; it owes its reputation to its great 
purity it contains a very small quantity of iron. 

In order to prepare alum quite free from iron from the 
ordinary alum containing iron, it is recrystallised, that is, as 
much 'as possible is dissolved in boiling water and the 
solution quickly cooled with coritinual stirring; the small 
crystals so obtained are then washed with cold water. The 
residual saturated solution of alum, which contains the 
greater part of the iron, can be used for the preparation of 
those colours which are not injured by the presence of iron. 

The solubility of alum in water varies greatly at different 
temperatures. The table gives the weight of alum dissolved 
by 100 parts of water at different temperatures. 



Temperature. 


Crystallised Alum. 


Anhydrous Alum. 


0C. 


3-90 


2-10 


10 


9-52 


4-99 


20 


15-13 


7-74 


30 


22-01 


10-94 


40 


30-92 


14-88 


50 


44-11 


20-09 


60 


66-65 


26-76 


70 


90-67 


35-11 


80 


134-47 


45-66 


90 


209-31 


58-64 


100 


357-48 


74-53 



46 MINEEAL AND LAKE PIGMENTS. 

When potash alum is heated it loses water, 75 per cent, 
of the total at 61 C. ; at 92 C. it melts completely, and all 
the water is lost by continued heating at 100 C. The residue 
is known as burnt alum. 

In alum the whole acidity of the sulphuric acid is not 
neutralised ; the solution has always an acid reaction ; if 
soda solution is added, the escaping carbonic acid causes the 
liquid to effervesce. If soda solution is added until a further 
addition would cause a precipitate, a solution of so-called 
neutral alum is formed which has no longer an acid reaction. 
Neutral alum is occasionally required in colour making. In 
preparing it the soda solution must be added with great care 
when the liquid is near its point of neutralisation. Any ad- 
dition of soda solution after this point is reached will cause a 
separation of alumina. This is not desirable, since it is gener- 
ally only wished to precipitate the alumina in combination 
with colouring matters. 

Roman Alum. Under this name, or that of " cubic alum," 
a variety of alum is sold, generally at a rather higher price 
than ordinary alum, from w 7 hiqh it is distinguished by its 
crystalline form. Ordinary alum forms octahedral crystals 
often the size of a child's head, but cubic alum well formed 
cubes. 

The property of crystallising in cubes may be imparted 
to any alum solution by the addition of a little potash. 
Much so-called Roman alum is made in German works in 
this way. When this alum contains very little iron it is quite 
equal in quality to the best Eoman alum, for the higher 
value of the latter is entirely due to its small content of iron. 
The alum prepared in the province of Naples is still better 
than Eoman alum ; it contains less iron. 

When alum is required for the preparation of lakes of 
bright and delicate shades, it is indispensable to use a pre- 
paration free from iron, because the brownish yellow oxide 



METALLIC COMPOUNDS. 47 

of iron would appreciably injure the shade. Alum, free from 
iron, is most simply prepared by dissolving alum in boiling 
water, running the boiling solution quickly through a cloth, 
and quickly cooling with constant stirring. The alum meal 
prepared in this way contains much less iron than the 
original alum, the iron salts remaining dissolved in the 
mother liquor. When this alum meal is collected and cold 
water poured over it to free it from mother liquor, it is 
generally sufficiently pure to be used for any purpose, but 
if not, it is again recrystallised. The alum liquors contain- 
ing the iron are used for the preparation of colours which 
are not injured by the presence of iron. 

To test alum for iron yellow prussiate of potash is used, 
which gives a blue precipitate with ferric salts. The test is 
carried 'out by dissolving 10 grammes of alum in 1 litre of 
water, placing the solution in a tall, narrow cylinder stand- 
ing on white paper, adding 10 to 20 drops of a saturated 
solution of yellow prussiate, and well stirring. On looking 
down through the liquid, if a distinct colouration is at once 
evident, the alum contains much iron, and must be recrystal- 
lised ; indeed, the crystals would generally be coloured yellow. 
On the contrary, if the solution does not show a blue tint 
until after standing several days, the alum contains but a 
small quantity of iron, and can be used for most purposes 
without further purification. Alum quite free from iron is 
a rare commercial article ; the test will generally show a 
feeble blue colouration. Tf this is not intense and no blue 
precipitate is deposited at the bottom, the alum is tolerably 
pure, and can be used in colour works. The longer the time 
before the blue colouration appears the poorer is the alum in 
iron. 

Soda Alum, NaAlfSOJ, . 12H/) = 458, is made in some 
alum works. It has the greatest similarity in properties with 
the potash salt, but is distinguished by a much greater solu- 



48 



MINEEAL AND LAKE PIGMENTS. 



bility in water and more rapid efflorescence in air. Soda 
alum can be bought at varying prices ; that containing iron 
is much cheaper than that free from iron. When the latter 
is to be bought at a fair price it is to be preferred to potash 
alum, since, as we shall show later, it contains a larger pro- 
portion of alumina. 

Ammonia Alum, (NH 4 )A1(S0 4 ) 2 . 12H,0 = 453. This com- 
pound of aluminium sulphate and ammonium sulphate is 
now often met with ; the more expensive potassium sul- 
phate in ordinary alum is replaced by ammonium sulphate, 
which is cheaply obtained from the ammoniacal liquor of the 
gas works. 

Ammonia alum is better for our purpose than potash 
alum since it contains more alumina, is generally cheaper 
and dissolves more easily in water. Unfortunately most 
commercial ammonia alum contains so much iron that it 
has to be recrystallised before it can be used in colour works. 

One hundred parts of water dissolve at different tempera- 
tures the quantities of ammonia alum given in the table : 



Temperature. 


Crystallised Ammonia Alum. 


Anhydrous Ammonia Alum. 


0C. 


5-22 


2-62 


10 


9-16 


4-50 


20 


13-66 


6-57 


30 


19-29 


9-05 


40 


27-27 


12-35 


50 


36-51 


15-90 


60 


51-29 


21-95 


70 


71-97 


26-09 


80 


103-08 


35-19 


90 


187-82 


50-30 


100 


421-90 


70-83 



Of the different alums, ammonia alum contains the largest 
and potash alum the smallest proportion of alumina. The 
composition of the three commonly occurring alums is given 
in the following table : 



METALLIC COMPOUNDS. 



49 





Potash Alum. 


Soda Alum. 


Ammonia Alum. 


Potash, K 2 .... 
Soda, Na^O .... 


9-95 


6-80 





Ammonia, NH. . 








3-89 


Alumina, A1 2 3 


10-83 


11-20 


11-90 


Sulphuric acid, S0 3 


33-71 


34-90 


. 36-10 


Water, H 2 4 .... 


45-51 


47-10 


48-11 



Thus ammonia alum is to be preferred to soda alum and 
soda to potash alum, whilst the latter is used on account of 
its greater purity. In a colour works, in which large quanti- 
ties of alum are used, it is advantageous to work with 
ammonia alum which is recrystallised on the works. 

The alums and aluminium sulphate are the alumina 
compounds in ordinary use in colour works ; aluminium 
acetate could also be used if it were to be had at a reasonable 
price. 

Alumina, A1 2 3 = 102, and Hydrate of Alumina. Pure 
alumina, or rather hydrate of alumina, is required in the 
preparation of many colours. When the solution of an 
aluminium salt is precipitated by soda, the carbonic acid 
escapes with effervescence, and a gelatinous precipitate is 
formed which it is extremely difficult to wash clean. The 
precipitate, which consists of hydrate of alumina, shrinks 
very greatly in drying, and turns to a horny mass ; when 
strongly heated it loses water, and becomes a white, insoluble 
powder of anhydrous alumina. 

A variety of hydrate of alumina, heavy, and therefore 
easily washed, is obtained by boiling a solution of alum with 
a plate of zinc lying on a copper plate until all the alumina 
has separated. By collecting this on a filter and pouring hot 
water over it a number of times it is obtained quite pure. 

Alumina plays a particularly important part in the manu- 
facture of cobalt colours. When treated with a cobalt salt 

and heated it takes a fine blue shade. 

4 



50 MINERAL AND LAKE PIGMENTS. 

In the foregoing those metals and their compounds have 
been treated which are extensively used in preparing colours 
without themselves forming coloured compounds. The chro- 
mates and prussiates are an exception to this. The ammonia 
compounds, the alkalis and alkaline earths, also the acids, are 
used in making many colours, although they do not contain 
colouring principles. The alumina compounds are in similar 
case. Themselves colourless, they form at the same time a 
carrier for the coloured compound and bring it into a suitable 
form for use as a pigment. 

An example will explain what we mean by "carrier" of 
the colouring matter. Logwood contains a very handsome 
colouring matter which can be extracted by water. In order 
to be able to employ this colouring matter as a pigment it 
is combined with alumina, a compound insoluble in water 
being formed, which is called a lake. In this compound the 
alumina is to be regarded as the carrier of the colouring 
matter, which it has fixed in an insoluble form. 

In dyeing, which in many respects is closely allied to 
colour making, the property of certain metallic compounds, 
themselves colourless, of fixing dyes is commonly utilised, 
the metallic compound being called the mordant. The fabric 
is first prepared with the metallic compound or mordant, 
and the colour then formed by bringing the mordanted 
material in contact with the colouring matter. 

The "heavy metals" form, among their numerous coin- 
pounds, a great number of coloured substances, and several 
of them are distinguished by a great wealth of coloured 
derivatives ; for example, copper, chromium and cobalt form 
coloured compounds only. Although the use of pigments 
derived from the heavy metals has been considerably re- 
stricted in recent years by the discovery of a series of 
colouring matters which replace them, yet they are now, 
and always will be, of very great importance in the manu- 



METALLIC COMPOUNDS. 51 

faeture of colours. It is, therefore, necessary briefly to 
describe the various metals which are used in colour 
making, so that the manufacturer may know what metals 
produce harmless colours, permanent and unaltered by the 
atmosphere* and what do not. 

The metals are divided into two great groups, designated, 
according to their specific gravities, the group of the light 
and of the heavy metals. The light metals comprise the 
alkali, alkaline earth and earth rnetals, whose important 
compounds we have ,just described. The specific gravity 
of each of these metals is less than five times that of 
water. 

The heavy metals have a specific gravity exceeding 5 ; 
they are generally divided into groups, which are known by 
the name of the commonest metal in the group. These are 
as follows :- 

Zinc group .... Zinc, Zn = 65. 

Iron group .... Iron, Fe = 56. 

Tungsten group . . . Tungsten, W = 184. 

Antimony group . . . Antimony, Sb = 120. 

Tin group .... Tin, Sn 119. 

Lead group .... Lead, Pb = 207. 

Silver group .... Silver, Ag = 108. 

Gold group .... Gold, Au - 197. 

Platinum group . . . Platinum, Pt = 194. 

To the zinc group belong the metals zinc, cadmium and 
indium, of which only the two first are of importance here. 
The iron group comprises iron, manganese, cobalt, nickel, 
chromium and uranium, all of which are used in the manu- 
facture of colours The antimony group contain^ antimony 
and bismuth, the latter of which is of little importance. To 
the tin group belong tin and the rare metals titanium, zir- 
conium, thorium, niobium and tantalum ; of these tin alone 
is important in colour making. In the lead group are lead 
and thallium ; lead produces many important pigments. The 



52 MINEEAL AND LAKE PIGMENTS. 

silver group contains silver, copper and mercury, the two- 
latter of which are important. Of the gold group gold alone 
is of interest, and its importance has been diminished by the 
discovery of far cheaper substances which replace it. In the 
platinum group, which contains platinum, iridium, rhodium, 
ruthenium, palladium and osmium, only platinum itself is 
used as a colour ; it is employed in porcelain painting to 
produce the peculiar metallic shimmer known technically as- 
"lustre." 

The behaviour of the compounds, of the metals mentioned 
above towards sulphuretted hydrogen is of the greatest im- 
portance to the colour maker, since 011 it depends the alter- 
ability of the pigments when exposed to the atmosphere. 
Many metallic compounds are unaltered by the sulphuretted 
hydrogen present in the air, whilst others are in a high 
degree affected by it and become gradually darker, so that 
their colour may in the end approximate to black. 

The pigments containing lead, copper, mercury and bis- 
muth are extremely susceptible to the action of sulphuretted 
hydrogen, by the action of which they form black compounds. 
Since colours which contain these metals are not permanent, 
but darken considerably, endeavours have been made for a, 
long time to replace them by others not susceptible to the 
action of sulphuretted hydrogen. Thus it is desirable to 
manufacture only colours free from metals forming black 
compounds with sulphuretted hydrogen. For the same 
reason pigments which contain sulphur should not be mixed 
with those containing metals which form black sulphur 
compounds. 

The rules laid down in the preceding paragraphs are of 
the greatest importance for the artist, for by following them 
he will succeed in composing a "permanent palette," that 
is, containing only such colours as will not, by their com- 
position, bring about the speedy ruin of the painting. 



METALLIC COMPOUNDS. 53 

Although the majority of the compounds of the heavy 
metals are poisonous, some possess this property in an 
eminent degree. These are chiefly colours which contain 
arsenic, antimony, copper and lead. So far as it is possible, 
these colours should be dispensed with and harmless pig- 
ments sold in their place, though this is not always possible, 
since several poisonous colours cannot be replaced by inno- 
cuous ones. 

COMPOUNDS OF THE HEAVY METALS. 

Zinc Compounds, Zinc oxide, ZnO, and zinc sulphate, 
ZnS0 4 .7H 2 O, are the compounds of this metal used in the 
colour industry. Zinc oxide, which is used as a white 
pigment, is a powder which turns yellow when heated, 
and is not acted upon by sulphuretted hydrogen. Zinc 
sulphate (white vitriol) occurs as colourless crystals, or more 
frequently as greyish white crystalline masses. The freedom 
of zinc sulphate from iron is of particular importance ; the 
commercial article is seldom satisfactory in this respect. 
In order to free commercial zinc sulphate from iron, the 
property of zinc hydroxide of precipitating iron oxide from 
neutral solutions may be employed. The zinc sulphate is 
dissolved in water, and ammonia added in small quantities 
until the precipitate of zinc hydroxide remains on stirring. 
When the liquid is left in contact with the precipitate and 
stirred up once or twice a day, if iron is present the pre- 
cipitate will turn yellowish brown, owing to the separa- 
tion of ferric hydroxide, and in the course of a few days 
all the iron will be removed from solution. The liquid 
should then give no blue colouration with yellow prussiate 
of potash. 

Zinc oxide is used as a white pigment, zinc chromate as 
a yellow, and zinc cobalt compounds as green colours. 

Cadmium Compounds. Cadmium is a metal which pos- 



54 MINEBAL AND LAKE PIGMENTS. 

sesses great similarity to zinc, with which it occurs in 
nature. In the preparation of cadmium compounds the 
metal is generally used. This is dissolved in dilute sul- 
phuric acid, hydrogen is evolved, and a solution of cadmium 
sulphate obtained. 

Cadmium is used in colour making only for the prepara- 
tion of the beautiful cadmium yellows. 

Iron Compounds. These are of the greatest importance 
to the colour maker. Several, in which iron alone is the 
colour principle, are very valuable : ochre, rouge, Venetian 
red, sienna and umber, for example. Iron compounds are 
also used in the preparation of many colours. The most 
important is : 

Ferrous Sulphate (Green Vitriol, Copperas), FeSO 4 .7H 2 O. 
This substance, which occurs commercially in a form of 
great purity at a very low price, is generally the starting- 
point in the preparation of iron pigments. When pure, it 
forms fine sea-green crystals, with an astringent metallic 
taste, which are not poisonous and are easily soluble in 
water. After long exposure to the air, ferrous sulphate 
becomes covered with an ochre-coloured crust, consisting 
of basic ferric sulphate. The ferrous oxide contained in the 
green vitriol has united with oxygen and been converted 
into ferric oxide. The latter requires a larger quantity of 
acid than ferrous oxide for the formation of soluble salts, so 
that an insoluble basic salt is separated. The same thing 
occurs when a solution of ferrous sulphate is exposed to 
the air. 

When green vitriol, or any other ferrous salt, is exposed 
to the action of oxidising agents, such as chlorine or nitric 
acid, the iron is rapidly changed into the ferric state. This 
transformation is of particular importance in the manufac- 
ture of certain blue pigments. 

We give in a table the relation between the percentage 



METALLIC COMPOUNDS. 



55 



of crystallised ferrous sulphate (FeSO 4 .7H 2 O) contained in 
a solution at 15 C. and its specific gravity : 



Specific Gravity. 


Percentage of Ferrous 
Sulphate. 


Specific Gravity. 


Percentage of Ferrous 
Sulphate. 


1-000 





1-118 


21 


1-005 


1 


1-125 


22 


1-011 


2 


1-131 


23 


1-016 


3 


1-137 


24 


1-021 


4 


1-143 


25 


1-027 


5 


1-149 


26 


1-032 


6 


1-155 


27 


1-037 


7 


1-161 


28 


1-043 


8 


1-168 


29 


1-048 


9 


1-174 


30 


1-054 


10 


1-180 


31 


1-059 


11 


1-187 


32 


1-065 


12 


1-193 


33 


1-071 


18 


1-200 


34 


1-077 


14 


1-206 


35 


1-082 


15 


1-213 


36 


1-088 


16 


1-219 


37 


1-094 


17 


1-226 


38 


1-100 


18 


1-232 


39 


1-106 


19 


1-239 


40 


1-112 


20 









Yellow and red prussiate., which have been already men- 
tioned, also belong to the iron compounds. They have been 
separately mentioned because the iron is contained in them 
in a peculiar form as a portion of an organic radical. 

Ferrous Chloride, FeCl. 2 , may sometimes be used instead 
of green vitriol. When iron is dissolved in hydrochloric acid 
hydrogen is given off and a solution of ferrous chloride is 
obtained ; but when rouge is dissolved in the same acid, ferric 
chloride is formed. When iron is dissolved in nitric acid, 
in consequence of the oxidising properties of this acid a 
ferric salt is obtained. Iron forms two series of salts : in the 
ferrous compounds the iron is in the same form as in green 
vitriol and the corresponding salts ; in the ferric compounds 
the iron is contained in a higher state of oxidation. By 
powerful oxidising agents, as nitric acid or chlorine, ferrous 
compounds are converted into ferric. 



56 MINERAL AND LAKE PIGMENTS. 

Manganese Compounds, Manganese (Mn) is a metal 
whose compounds show great similarity with those of iron, 
like which it forms two oxides (also others), manganous 
oxide (MnO) and manganic oxide (Mn 2 3 ). The salts of 
manganous oxide are not oxidised in the air like those of 
ferrous oxide. 

The raw material used in the preparation of manganese 
compounds is the mineral pyrolusite, which is manganese 
dioxide (Mn0 2 ). 

Manganese sulphate (MnS0 4 ) forms rose-red crystals 
containing varying quantities of water. The residues from 
the preparation of chlorine can be used as the material for 
the preparation of colours. According as pyrolusite and 
hydrochloric acid or pyrolusite, salt and sulphuric acid are 
used for this purpose a solution of manganous chloride or 
sulphate is obtained. 

Manganese compounds have but a restricted use in colour 
making. 

Nickel Compounds are generally coloured green, but they 
are not used as pigments. 

Cobalt Compounds. Among these are found many im- 
portant pigments. All cobalt compounds are coloured ; in 
beauty and variety of shade they can only be compared 
with those of chromium. In properties cobalt is very 
similar to iron and nickel. 

The form in which cobalt is used in preparing colours 
is either cobalt nitrate, Co(N0 3 ) 2 .6H 2 0, or cobalt chloride, 
CoCL .6H..O. Both salts are articles of commerce, but 
generally they are so dear that it is more profitable for the 
colour maker to prepare them direct from the cobalt minerals. 
A simple method for preparing cobalt compounds from the 
ores is therefore given. The most important cobalt ores are 
speiss cobalt, a compound of cobalt and arsenic, and cobalt 
glance, a compound of cobalt, arsenic and sulphur. The 



METALLIC COMPOUNDS. 57 

former mineral often contains only small quantities of 
cobalt, and it is advisable for our purposes to use cobalt, 
glance, which contains from thirty to forty per cent, of 
cobalt. This mineral is first roasted, that is, is heated with 
a plentiful air supply, by which means the arsenic is driven 
off. On account of the poisonous nature of the arsenic 
vapours the roasting must be conducted in a furnace with 
a very good draught. 

Under the name of zaffre, roasted cobalt ores come into 
commerce. These may be used in the preparation of cobalt 
compounds, by which means the operation of roasting is 
avoided. According to the quality of the ore which has 
been used to obtain zaffre, it contains a very varying pro- 
portion of cobalt. The varieties richer in cobalt must be 
used; they are technically known by the mark FS, or 
FFS (the best). 

The roasted cobalt ores or zaffre are treated with fused acid 
potassium sulphate, when the salts of iron and manganese are 
decomposed, whilst cobalt and nickel sulphates remain un- 
changed. In a Hessian crucible are melted 300 parts of 
acid potassium sulphate, and 100 parts of the powdered 
zaffre are gradually added, mixed with one part of green 
vitriol and one part of saltpetre ; the mixture is heated so 
long as sulphuric acid escapes. The mass is then boiled 
with water and the red solution treated with sulphuretted 
hydrogen so long as a precipitate is formed ; this may 
contain copper, manganese, and bismuth. After filtering, 
soda is added to the boiling liquid ; cobalt carbonate is pre- 
cipitated, which can be converted into nitrate or chloride 
by solution in the corresponding acid. If cobalt sulphate 
is required, the solution, after treatment with sulphuretted 
hydrogen, need only be evaporated to crystallisation, when 
the sulphate separates in fine red crystals. Cobalt nitrate 
and chloride are very soluble in water ; to obtain them their 



58 MINERAL AND LAKE PIGMENTS. 

solutions must be strongly evaporated and quickly cooled 
whilst stirring. The crystals of cobalt nitrate and chloride 
absorb moisture from the air and deliquesce ; they must be 
kept in glass vessels with well-ground stoppers. 

The cobalt compounds which are to be used in colour 
making must be free from iron, nickel and arsenic, which 
would detract from the cleanness of the colours. If the 
precipitate produced by soda contains iron it is mixed with 
excess of solution of oxalic acid, and after a few hours the 
cobalt oxalate is filtered from the liquid, in which all the 
iron is dissolved. The cobalt oxalate can then be converted 
into nitrate or chloride by treatment with nitric or hydro- 
chloric acids. 

These salts form the material for the preparation of the 
cobalt compounds, a large number of which are used as 
extremely durable red, blue and green pigments ; several of 
them, such as cobalt blue, cannot be exactly replaced by 
other pigments. On account of the industrial importance 
of the cobalt colours, these directions for the preparation of 
the soluble cobalt salts from the ores have been given with 
some detail. The preparation of the cobalt colours will be 
given in extenso later on. 

Chromium Compounds As the name indicates, this 

metal yields numerous coloured compounds (x/ow/Aa, colour) ; 
in fact, only coloured chromium compounds are known, and 
the colours are most varied yellow, green, red and violet. 
On this account the chromium compounds are among the 
most important used in colour making ; a great number of 
colours are prepared by their aid. Chrome ironstone, as we 
have already stated, is the raw material for the preparation 
of chromium compounds. From it potassium bichromate is 
made on a large scale in special works, so that no colour 
maker is compelled to prepare chromium salts himself. 

When the chromium pigments contain no metal black- 



METALLIC COMPOUNDS. 59 

ened by sulphuretted hydrogen, they have the desirable pro- 
perty of being unaltered by the atmosphere. Like the cobalt 
compounds, they are distinguished by their great stability 
when heated ; on this account, they have a large use in 
porcelain painting. 

Molybdenum, Tungsten and Vanadium Compounds on 
account of their cost have a very limited use as pigments. 
Molybdenum compounds are obtained from molybdic acid ; 
compounds of tungsten from the metal ; and those of vana- 
dium from ammonium vanadate. 

Antimony Compounds can be used in the preparation of 
several pigments, but, on account of their behaviour towards 
sulphuretted hydrogen, the pigments cannot be regarded as- 
really permanent, and their use is generally diminishing. 
The so-called antimony vermilion is the only antimony com- 
pound at all extensively employed. 

Bismuth Compounds possess properties very similar to 
those of antimony. Only one bismuth preparation is used 
as a pigment, and this is very sensitive to the action of 
sulphuretted hydrogen, being changed into black bismuth 
sulphide. 

Tin Compounds are employed in two ways : some are 
themselves colours, such as stannic sulphide (mosaic gold) ;. 
others, themselves colourless, are used in making pigments, 
as stannous and stannic chlorides. 

Stannous Chloride, SnCl 2 .2 H 2 O, is obtained when tin is 
dissolved in hydrochloric acid, hydrogen being evolved. 
Stannic Chloride, SnCl 4 , is formed when tin is dissolved in 
a mixture of hydrochloric and nitric acids (aqua regia). 

Tin compounds have, similarly to aluminium salts, the 
property of forming coloured insoluble compounds (lakes) 
with many organic colouring matters. Their use for this 
purpose is extensive. 

Arsenic Compounds formerly played an important part 



MINERAL AND LAKE PIGMENTS. 

in colour making. They were used in the manufacture of 
a large number of pigments, very beautiful but extremely 
poisonous. At the present time, we can, fortunately, entirely 
dispense with arsenic in colour making ; the arsenic colours 
can be replaced by others equally handsome and less, or not 
at all poisonous. The most important of the arsenic com- 
pounds is the trioxide As 2 3 , or Arsenious Acid, commercially 
known as white arsenic. This substance is obtained in 
large quantities as a by-product in the extraction of several 
metals. It forms masses which are either glassy or have 
the appearance of porcelain. Freshly sublimed arsenic 
trioxide is glassy ; this form gradually changes into the 
porcellaneous variety, it dissolves with difficulty in water. 
A strong solution can only be obtained by boiling for many 
hours. 

The compounds of arsenic with sulphur, formerly exten- 
sively used as pigments, have now almost fallen into disuse. 

Lead Compounds belong to the substances most largely 
used in making colours. Unfortunately all lead colours 
have two very important drawbacks. They are all very 
poisonous and at the same time extremely sensitive to sul- 
phuretted hydrogen, so that they are very considerably 
altered by the action of the small quantities of that gas 
contained in the atmosphere of an ordinary dwelling. A 
striking example of this is seen in the lead paints used on 
the doors of water-closets. The paint, at first pure white, 
becomes gradually darker, and at last almost black, the lead 
compound having been changed into black lead sulphide. 

On account of this great sensitiveness of lead compounds, 
it would be better if they could be excluded from the list of 
colours. Great care must be taken not to mix lead com- 
pounds with others which contain sulphur ; a discolouration 
of the mixture would be the inevitable result in a very short 
.time. 



METALLIC COMPOUNDS. 61 

The oxides of lead and a number of its salts are them- 
selves pigments, for example, litharge, PbO, red lead, Pb 3 O 4 , 
and white lead (basic lead carbonate). These pigments are 
prepared on a large scale in particular works. At this point 
only those lead compounds will be mentioned which are 
generally used for the preparation of other lead pigments ; 
they are : lead sulphate, nitrate, acetate and chloride. 

Lead Sulphate, PbSO 4 , is formed when sulphuric acid or 
the solution of a sulphate is added to the solution of a lead 
salt ; so obtained it is a white crystalline powder insoluble in 
water. This substance is generally not made in colour works, 
but is purchased from chemical works or dye houses, of which 
it is a by-product. In this form (lead bottoms) it is generally 
not sufficiently pure, but contains admixtures of sulphuric 
acid or aluminium salts, from which it is freed by washing. 
The lead sulphate is stirred up in water, the heavy precipi- 
tate allowed to settle, the wash water drawn off, and after 
repeating this process until the wash water no longer shows 
an acid reaction, the purified precipitate is dried. In this 
condition it is a heavy, white powder, and can alone be 
ground into paint. But on account of its crystalline nature, 
which reduces the covering power, such use is inadvisable. 

Lead Nitrate, Pb(N0 3 ) 2 . This very important com- 
pound may be bought, but it is advisable to prepare it in 
the works. Water is placed in a wooden tub, then half the 
volume of nitric acid is added, and finely powdered litharge 
gradually stirred in, the liquid being kept in constant move- 
ment. When it is seen that the litharge is only slowly dis- 
solved, the liquid is well stirred after each addition of litharge 
and then tested by litmus paper. When this is no longer 
reddened, the nitric acid is completely saturated, and the 
liquid contains only lead nitrate in solution. It is allowed 
to stand until the insoluble portions have settled, and then 
drawn off into another vessel where crvstals of lead nitrate 



62 MINERAL AND LAKE PIGMENTS. 

separate in a few days. If the salt be required in solid 
form, the solution may be evaporated in earthenware dishes ; 
generally the solution is used as it is obtained. 

Pure lead nitrate forms white crystals which are not 
particularly soluble in water, 1 part requiring 2 parts of 
water at the ordinary temperature. Lead nitrate is decom- 
posed on heating, like all nitrates, and litharge remains. 
The solution of this salt is used in the preparation of those 
lead pigments which are obtained by precipitation, for 
example, chrome yellow. 

Lead Acetate, Pb(C 2 H 8 2 ) 2 .3H 2 O. The compounds of 
lead with acetic acid are of great importance. Two of these 
are to be considered : neutral lead acetate, commonly known 
as sugar of lead, and basic lead acetate. It may be advisable 
to manufacture both these compounds, the latter always. 
Neutral lead acetate comes into commerce in the form of 
colourless heavy crystals, which are often covered by a white 
powder of the basic acetate ; they dissolve readily in water, 
and the solution has a sweetish taste, hence the name " sugar 
of lead ". Frequently the solution is very turbid ; this is 
caused by the carbonates contained in the water. The tur- 
bidity may be removed by the addition of a little acetic acid. 
It is only economical for the colour maker to prepare sugar 
of lead when he can obtain cheap raw materials, lead or 
litharge and vinegar. Pyroligneous acid may also be used 
if it is colourless, its odour being without importance for this 
purpose. 

The best method for preparing lead acetate from litharge 
is to place the vinegar in a tub ' and hang in it a strong linen 
bag filled with finely-ground litharge. The tub is kept 
covered for a few days and its contents then tested with red 
litmus paper. When this is turned blue, the liquid is drawn 
off and vinegar gradually added whilst stirring, until blue 
litmus paper is just turned red. In this process, after neutral 



METALLIC COMPOUNDS. 63 

lead acetate has been formed, more lead oxide is dissolved, 
and the liquid thus acquires an alkaline reaction. The 
further addition of acetic acid reconverts the basic salt into 
neutral lead acetate. 

Lead acetate solution may, with advantage, be prepared 
directly from metallic lead. For this purpose, lead is granu- 
lated by melting and pouring in a thin stream into cold water, 
where it solidifies in irregular pieces. This is done in order 
to give the lead as large a surface as possible. Three high 
narrow tubs placed one above the other so that liquid may flow 
from the highest to the middle, and from this into the lowest, 
are filled with granulated lead. Vinegar is placed in the upper- 
most vessel to cover the lead ; after twenty-four hours it is 
allowed to flow into the middle, and after a further twenty- 
four hours into the lowest tub. In this way, a solution of 
basic lead acetate is formed, to which the necessary quantity 
of acetic acid is added to bring it into the neutral condition. 
If crystalline lead acetate is required, the liquid is evaporated 
down and quickly cooled with stirring, so that small crystals 
are formed. Generally, however, evaporation is unnecessary, 
since lead acetate is always used in solution in preparing 
colours. 

If the lead acetate solution be not colourless, which is 
generally the case when coloured acetic acid is used, the 
defect may be removed by stirring a little bone black into 
the liquid and filtering after twenty-four hours, when a com- 
pletely colourless solution is obtained. 

It is always necessary to know exactly how much lead 
acetate is contained in the solutions prepared by these pro- 
cesses. The lead or the litharge is therefore weighed and 
the volume of the lead acetate solution measured. One 
hundred parts by weight of crystallised lead acetate are 
obtained from 62'54 parts of lead. 

Lead acetate solutions must be kept in well-covered 



64 



MINEKAL AND LAKE PIGMENTS. 



vessels. The carbonic acid of the air will turn the liquid 
turbid. The turbidity may be removed by the addition of 
acetic acid. 

In the following table is given the percentage of crys- 
tallised lead acetate contained in solutions of different specific 
gravities : 



Specific 
Gravity. 


Crystallised 
Lead Acetate 
per cent. 


^nwifir. i Crystallised 

ssft in^r 


Specific 
Gravity. 


Crystallised 
Lead Acetate 
per cent. 


1-0000 





1-1159 17 


1-2578 


34 


1-0064 


1 


1-1234 18 


1-2669 


35 


1-0127 


2 


1-1309 19 


1-2768 


36 


1-0191 


3 


1-1384 20 


1-2867 


37 


1-0255 


4 


1-1464 21 . 


1-2966 


38 


1-0319 


5 


1-1544 22 


1-3064 


39 


1-0386 


6 


1-1624 23 


1-3163 


40 


1-0453 


7 


1-1704 24 


1-3269 


41 


1-0520 


8 


1-1784 25 


1-3376 


42 


1-0587 


9 


1-1869 , 26 


1-3482 


43 


1-0654 


10 


1-1955 27 


1-3588 


44 


1-0725 


11 


1-2040 28 


1-3695 


45 


1-0796 


12 


1-2126 29 


1-3810 


46 


1-0867 


13 


1-2211 30 


1-3925 


47 


1-0939 


14 


1-2303 31 


1-4041 


48 


1-1010 


15 


1-2395 32 


1-4156 


49 


1-1084 


16 


1-2486 33 

| 


1-4271 


50 



Basic Lead Acetate, Pb(C 2 H 3 2 ) 2 .2PbO. -- This salt 
may be regarded as a compound of neutral lead acetate 
with lead oxide. It is obtained by digesting vinegar with 
excess of litharge or with metallic lead, and also by treat- 
ing lead acetate solution with litharge so long as the latter 
is dissolved. In the method last given, 100 parts of sugar 
of lead require about 118 parts of litharge to produce a satu- 
rated solution of basic acetate. The solution of this com- 
pound is alkaline ; it turns red litmus paper blue. When 
exposed to the air, a turbidity is quickly produced owing to 
the separation of lead carbonate. In one white lead process 
basic lead acetate is the starting-point of the manufacture. 

Lead Chloride, PbCl 2 , is seldom used in making colours. 
It may be prepared by stirring powdered litharge in common 



METALLIC COMPOUNDS. 65 

salt solution until the powder appears white. This, when 
washed, constitutes basic lead chloride. On adding hydro- 
chloric acid to the washed mass until the liquid remains 
acid, lead chloride is obtained in the form of crystalline 
needles, which are very little soluble in cold, but more easily 
in hot, water. 

Like any other soluble lead salt, lead chloride may be 
used in the precipitation of colours, but is seldom employed 
on account of its small solubility. Basic lead chloride was, 
at one time, used as a white pigment, and after melting, by 
which it is turned yellow, as a yellow pigment ; it is no 
longer in use for these purposes. 

Copper Compounds, These are generally green or blue, 
and have an extended use in the production of colours. The 
metallic copper which is used in the preparation of colours 
is of the ordinary commercial quality. The impurities which 
it contains are generally so small in quantity that they are 
without importance for our purpose. 

Copper Sulphate (Bluestone, Blue Vitriol), CuS0 4 .5H 2 0. 
This is the commonest of the commercial copper salts, and 
on that account deserves our especial attention. It forms 
large sky-blue crystals, which effloresce slightly in the air, 
possess an unpleasant metallic taste, and are poisonous, like 
all soluble copper compounds. 

Copper sulphate comes into commerce in a very pure 
form, but some qualities contain zinc sulphate or ferrous 
sulphate. The presence of zinc may be detected most 
easily by boiling the solution with excess of caustic soda, 
when copper oxide separates as a black powder, whilst zinc 
oxide remains dissolved. When sulphuretted hydrogen is 
passed through the liquid, a white precipitate of zinc sul- 
phide is formed. 

Iron is detected by passing sulphuretted hydrogen through 
the solution so long as a precipitate is formed, allowing the 

5 



66 MINERAL AND LAKE PIGMENTS. 

liquid to stand in a covered vessel, pouring it off from the 
precipitate, adding nitric acid, boiling and adding a solution 
of potassium ferrocyanide ; a blue precipitate denotes the 
presence of iron. 

Copper sulphate is rarely found which is quite free from 
iron and zinc. If these impurities are present in but small 
quantity, the zinc not exceeding 1 per cent, and the iron at 
most 0'5 per cent., the copper sulphate may be regarded as 
sufficiently pure for our purposes. 

It may be here remarked that copper sulphate obtained 
from mints is generally of great purity, and hence particu- 
larly adapted for colour making. 

When copper sulphate and other copper salts are dis- 
solved in water, pale blue flocks of copper carbonate generally 
separate. This is due to the carbonate of lime contained in 
the water. An addition of a few drops of sulphuric, nitric 
or hydrochloric acid suffices to prevent this separation. 

Copper Nitrate, Gu(N0 3 ) 2 .6H 2 0. This salt may occa- 
sionally be obtained in colour works as a by-product. When 
nitric acid is poured over copper, there follows a copious 
evolution of nitric oxide, which produces brown fumes of 
nitrogen peroxide in air. Nitric oxide may be used to 
convert ferrous into ferric salts, a transformation required 
in making Prussian blue. In working in this way, nitric 
acid is poured over copper contained in a vessel provided 
with a delivery tube for the gas. The blue solution is at 
once used. Pure copper nitrate forms fine blue crystals, 
which very readily deliquesce in the air. The solution is 
therefore generally used as it is prepared. 

Copper Acetate, Cu(C 2 H 3 O 2 } 9 . H. 2 O. Copper is readily 
attacked by acetic acid. A number of salts are formed, of 
which some are used as pigments. For our purpose it will 
be sufficient to describe the manufacture of verdigris ; few 
colour makers prepare any other copper acetate. A solution 



METALLIC COMPOUNDS. 67 

of this salt is most simply prepared in the following manner: 
Slaked lime is stirred with strong vinegar, and the solution 
left in contact with the excess of lime so long as it has a 
weak acid reaction. The solution, which contains acetate 
of lime, is poured into a splution of copper sulphate so long 
as a precipitate of sulphate of lime is formed. When this 
has been separated from the liquid, the latter is ready for 
further treatment. It contains only a very small quantity 
of dissolved sulphate of lime, which is not harmful in the 
preparation of colours. 

In addition to the copper compounds mentioned here, 
several others were formerly used as pigments, or in the 
preparation of pigments which are no longer employed, 
because copper compounds of good colour can be obtained 
in a cheaper manner. 

The same precautions should be taken in the use of 
copper colours which were mentioned for lead pigments ; 
copper compounds are equally sensitive towards sulphur- 
etted hydrogen, by which they are gradually discoloured. 

Mercury Compounds, Mercury forms compounds which, 
vermilion in particular, are used as pigments, and others 
which are used in the preparation of pigments. In many 
cases metallic mercury is the starting point in the pre- 
paration of the mercury compounds. The compounds 
commonly known as calomel and corrosive sublimate are 
also used. 

Mercurous Nitrate, HgNO a . Nitric acid acts upon 
mercury in a manner differing according to its strength, 
and according to whether the mercury or the nitric acid is 
used in excess. In order to prepare mercurous nitrate, acid 
free from chlorine must be diluted at least with four times 
its volume of water, and the mercury must be in excess. On 
warming, the mercury is gradually dissolved, and, on cooling, 
the solution deposits colourless crystalline needles of the salt. 



68 MINERAL AND LAKE PIGMENTS. 

A further crop of crystals is obtained after evaporating the 
solution. 

When the action of nitric acid is over, the solution must 
be at once separated from the excess of mercury to prevent 
the formation of basic salts. If the salt has been properly 
made, it is completely soluble in water, but if a lemon yellow 
precipitate is formed on dissolving, the nitrate contains a 
basic salt, which can only be dissolved by warming and 
adding more nitric acid. 

Mercuric Nitrate, Hg (N0 3 ) 2 , is most simply obtained by 
warming mercury with very strong nitric acid. The heating 
must be continued until a test portion of the solution no 
longer gives a precipitate with hydrochloric acid. When this 
solution is evaporated, nitric acid is given off, and a salt 
crystallising in white needles is obtained, which dissolves 
in water with the separation of a yellow basic salt. It is 
therefore better to use the hot solution, which contains a 
little free acid, without evaporating. 

Instead of mercurous and mercuric nitrates, the corre- 
sponding sulphates may be used, but the chlorides are more 
frequently employed since they can be readily obtained from 
the makers. 

Mercurous Chloride (Calomel), HgCl, is obtained pure by 
adding common salt solution to a solution of mercurous ni- 
trate and washing the precipitate, which is insoluble in water. 

Mercuric Chloride (Corrosive Sublimate), HgCl 2 , a common 
article of commerce, is prepared by heating a carefully 
made mixture of mercuric sulphate and common salt, when 
mercuric chloride sublimes. It is a white crystalline mass, 
soluble in 13*5 parts of water at 20 C., and soluble in 3 
parts of alcohol. Although all mercury compounds are very 
poisonous, corrosive sublimate requires particular care in 
handling, since its easy solubility makes it surpass all other 
mercury compounds in poisonousness. 



METALLIC COMPOUNDS. 69 

The mercuric sulphate required in the above preparation 
is obtained by heating mercury with sulphuric acid. Corro- 
sive sublimate can also be prepared by adding hydrochloric 
acid to mercurous nitrate, and heating, with gradual addi- 
tion of hydrochloric acid until a clear solution is formed, from 
which mercuric chloride crystallises on cooling. 

Silver Compounds, Silver nitrate, AgNO.j, is the only 
one of importance here. It is obtained by dissolving silver 
in nitric acid, when a blue solution is obtained because com- 
mercial silver contains copper, evaporating the solution to 
dryness, melting the residue, and keeping it molten until all 
the copper nitrate is decomposed. This point is recognised 
when a small portion of the melt dissolved in water does not 
give a blue colouration with excess of ammonia. Fused 
silver nitrate forms a white crystalline mass readily soluble 
in water and turning black when exposed to light, like many 
other silver compounds. 

Gold Compounds. Gold is now very little used in pre- 
paring colours. The compound used for this purpose is gold 
chloride, AuCl 3 , which is obtained by heating gold with 
hydrochloric acid, and adding nitric acid in small quantities 
until all the metal is dissolved. By careful evaporation of 
the yellow solution gold chloride is obtained in brownish 
yellow crystals, which easily dissolve in water. 

The compounds of molybdenum, vanadium and uranium 
are less used than those of gold, yet these metals find a 
srpecial use in the preparation of colours for porcelain paint- 
ing and for colouring glass, for which they are of great 
importance. The preparation of their compounds from the 
raw materials is complicated and not remunerative to the 
colour maker ; they should be obtained from chemical works. 

In the foregoing, the most important compounds of in- 
organic origin used in making colours have been briefly 



70 MINERAL AND LAKE PIGMENTS. 

described, less in order to teach the methods for their 
preparation than to give the manufacturer the means of 
learning their properties. Since the great development of 
chemical industries during the last decades it is more advan- 
tageous for the colour maker in most cases to draw his supply 
of these substances from works of good reputation than to 
make them himself; only in the case of a few substances, 
which are sold at unreasonably high prices, will it be profit- 
able for him to prepare them himself. 



CHAPTEK VI. 

THE MANUFACTURE OF MINERAL PIGMENTS. 

BY mineral pigments we understand those which consist of 
compounds of metals with elements such as sulphur, chlorine 
and iodine, or with compound radicals, such as cyanogen, 
or of metallic salts. 

Looking at the classification of mineral pigments from 
the chemical standpoint, a grouping according to the con- 
stituents would appear preferable, and we should have groups 
of colours consisting of metallic oxides, sulphides, salts, etc. 
In such a division of pigments, according to their consti- 
tuents, no regard would be taken of the colour of the pigment. 
The white zinc oxide would be placed in the same group 
with yellow lead oxide and red lead oxide ; and red mercury 
sulphide and yellow cadmium sulphide would fall into the 
same group of the sulphur compounds. 

The chemical composition of a pigment is of less import- 
ance to the colour manufacturer than its shade, and it 
therefore appears to us more reasonable to prefer to classify 
mineral pigments according to similarity of colour. We 
shall therefore place all those mineral pigments together 
which possess the same colour. 

Common usage differs from the scientific in the descrip- 
tion of colours. In the physical sense, yellow, red and blue 
are the so-called "simple colours," between which lie orange, 
green and violet as " mixed colours ". Physics knows no white 



72 MINERAL AND LAKE PIGMENTS. 

colour and no black colour, but describes white as a mixture 
of all the simple colours and black as the absence of colour. 
A grey or brown shade, produced by different mixtures of 
simple colours, is just as little known in the scale of colours 
as white or black. 

The colour maker follows, as we have said above, the 
common manner of speaking ; to him white and black are 
equally as much colours as red and green. Besides the pure 
principal colours (yellow, red and blue), and the mixed colours 
obtained from them (orange, green and violet), colour makers 
distinguish many shades of each colour lemon yellow, 
sulphur yellow, cherry red, blood red, violet blue, etc. For 
the present purpose it is of great importance to accurately 
distinguish the several shades, for the value of many colours 
is in proportion to their beauty of shade. The colour maker 
is often required to produce a colour of some particular shade, 
which he accomplishes in many cases by a suitable altera- 
tion in the process by which the colour is made, in other 
cases by mixing different colours, in which event chemistry 
is of no help to him ; he must depend on the sensitiveness of 
his eyes to colour. 

White Mineral Pigments, We are acquainted with a 
great number of white or, more properly, colourless mineral 
compounds ; they possess the property of reflecting, unde- 
composed, all the rays of light which fall upon them, in 
consequence of which they produce that impression upon 
the eye which we call white. According as a white sub- 
stance reflects every ray of light or absorbs a portion, we 
see it as a brilliant pure white or, in the latter case, as a 
white with a grey tinge. If a white body reflects the 
majority of the rays of light falling on it, but decomposes a 
small number, we perceive a white which has a yellow, blue 
or red tinge. 

The most valuable white for the colour maker is evidently 



THE MANUFACTURE OF MINERAL PIGMENTS. 73 

that which reflects, unaltered, all the rays of light ; it is the 
most brilliant, and free from eve^y tinge of colour. The 
physical condition of the substance is most important. Solid 
substances are either crystalline, that is, possess definite 
shapes formed according to a regular law, or they are 
amorphous, that is, are composed of irregularly formed 
particles. Snow and white lead may serve as representa- 
tives of these two classes. Snow is composed of small 
colourless crystals of ice, the flat surfaces of which reflect, 
undecomposed, the light falling on them. The smaller are 
the crystals, the more pure appears to us the whiteness of 
the snow, and the thinner is the layer of snow required to 
produce the sensation of whiteness. But if the snow crystals 
are larger, the white appears to have a bluish tinge, and only 
a thick layer of snow is opaque. White lead, being an amor- 
phous substance in a condition of very fine division, reflects 
the light very regularly, so that a thin layer of white lead 
appears quite opaque. 

Among the artificial pigments, crystalline or amorphous, 
exactly the same conditions hold good as between snow and 
white lead. Of amorphous pigments a very thin layer is in 
most cases sufficient to make the surface upon which they 
are spread invisible, or, as the technical expression runs, " to 
cover," whilst crystalline substances possess a smaller cover- 
ing power. A striking example of this is seen by a comparison 
of two white pigments, white lead and " patent white " (lead 
oxychloride). The former is amorphous, the latter crystalline. 
Both are completely colourless and reflect white light, but in 
consequence of its amorphous condition and finer particles, 
white lead possesses far greater covering power than " patent 
white ". 

Among all other colours the same rule holds. Amor- 
phous pigments have always a greater covering power than 
crystalline. The smaller the crystals of the latter the greater 



74 MINERAL AND LAKE PIGMENTS. 

is their covering power, so that in preparing pigments of a 
crystalline character care must be taken to make the crystals 
as small as possible. 

From the above definition of white pigments it follows 
that an immense number must exist, since every colourless 
substance in a state of fine division appears white. Gener- 
ally only those bodies are used which are insoluble in water, 
or almost insoluble, and which possess great covering power.. 
The following may be mentioned as white pigments, only a, 
few of which are in use : white lead, white zinc, permanent 
white, lead oxychloride, lead sulphate and sulphite, zinc 
oxychloride, lead antimoniate, antimony white, tin white,, 
tungsten white, and in addition certain earths, pipe clay, china 
clay, etc. Several of these pigments are far too expensive 
for ordinary use, and have no advantage over much cheaper 
pigments except for very special purposes, such, for example,, 
as bismuth white for cosmetics. 

In general use we find very few artificial white pigments ; 
these are lead, zinc and barium compounds. Circumstances 
may arise which make it expedient for the colour maker to 
manufacture other white pigments, for example, a demand 
for them, or favourable opportunities for obtaining the 
requisite raw materials. 

The white lead pigments, of which there is a large 
number, aS we have indicated, all have the great disadvan- 
tage that they are not permanent, that is, are changed by 
atmospheric influences. It is well known that lead is a very 
delicate reagent for sulphuretted hydrogen, with the sulphur 
of which it forms a black compound. Now the air, especi- 
ally in towns, contains sulphur in the form of sulphuretted 
hydrogen or ammonium sulphide ; though the quantity is 
very small, the fate of every white or coloured lead pig- 
ment is 'decided by it ; after a longer or shorter time it 
will be discoloured, will gradually darken, and finally be^ 



THE MANUFACTURE OF MINERAL PIGMENTS. 75 

turned black. In spite of this great changeableness of lead 
pigments they are used by artists and painters, although the 
majority could be replaced by more permanent pigments 
entirely unaltered by the atmosphere. 

WHITE LEAD. 

This pigment, which in addition to other good properties 
has remarkable covering power, was amongst the earliest 
known artificial pigments. Already in the fourth century 
before Christ, Dioscorides described the preparation of white 
lead, which was obtained by exposing lead to the action of 
the vapours of vinegar, removing the white layer and treat- 
ing it with water. The Roman writers describe a similar 
method ; they use the name cerussa, under which white lead 
is known to-day in commerce. 

Although white lead has been so long known, it was left 
to Bergmann, in 1774, to show that it contained carbonic 
acid ; before that it was believed to be lead acetate. The 
development of analytical chemistry was followed by a 
knowledge of its constitution and the use of more rational 
methods of manufacture. Whilst in the middle ages the 
manufacture of white lead was almost exclusively in the 
hands of the Dutch and Venetians, in later years it gradu- 
ally spread, and now many works are concerned in the 
manufacture of this pigment. That adulterations of white 
lead were not rare in former times appears from the writings 
of Basil Valentine, an alchemist of the fifteenth century. 

Commercially white lead is known under most varied 
titles, of which the following are the principal : White lead, 
Venetian white, Dutch white, Krems white, Kremnitz white, 
flake white, etc. 

According to its chemical composition, white lead is a 
compound of lead carbonate and lead hydroxide, that is. a basic 
lead carbonate. Commercial white lead, apart from inten- 



76 MINERAL AND LAKE PIGMENTS. 

tional admixtures of other white substances, contains lead 
carbonate and lead hydroxide in varying proportions, as is 
shown by the following analyses by Mulder, who found that 
all the samples examined by him were composed according 
to one or other of the following formulae : 

2 PbCO,. Pb(OH) 2 containing 86-27 per cent, of PbO. 
5 PbCO r> . 2 Pb(OH),, 85- 86 

3 PbCOJ. Pb(OH) 2 " 85-45 

4 PbCO 3 . Pb(OH) 2 85-00 

According to Hochstetter, the manufacture of white lead 
must be directed to obtaining the compound 2 PbC0 3 .(OH) 2 , 
which possesses the following percentage composition : 

PbO . . . 86-32 
C0 2 . . . 11-36 
H 2 O . . . 2-32 

The compound of this composition is distinguished by 
being completely amorphous, and so possesses the greatest 
covering power. Commercial white leads, as is seen from the 
formulae of Mulder, may differ appreciably from this com- 
position, w r hen they will have a smaller covering power, 
since they will contain some quantity of neutral lead car- 
bonate, PbCO 3 (containing 83*46 per cent, of PbO), which is 
crystalline. 

White lead is prepared according to very different 
methods, the principle of which consists in subjecting a 
solution of tribasic lead acetate to the action of carbonic 
acid, by which the basic carbonate is produced and the 
neutral acetate formed, the latter being then reconverted 
into the basic acetate, which again serves to produce white 
lead and so on. 



CHAPTEK VII. 

THE MANUFACTURE OF WHITE LEAD. 

THE processes by which white lead is or was manufactured 
may be divided, according to the principal operations, in the 
following manner : 

1. Processes in which metallic lead is subjected to the 
action of acetic acid vapour, whilst the vessel in which this 
operation is conducted is exposed to a higher temperature. 
In the oldest so-called Dutch method this increase of tem- 
perature is effected by the decomposition of manure, by 
which the vessels containing the lead and acetic acid are 
surrounded. In consequence of the heat produced by this 
fermentation, acetic acid and water are volatilised, and oxygen 
also being present, lead acetate is formed. 

Also, as a consequence of the oxidation of the lead, heat 
is produced, which accelerates the process, and lead oxide is 
formed in large quantity, which unites with the neutral 
acetate already formed to produce a basic compound. The 
vessels in which this process is taking place are in an atmo- 
sphere containing much carbonic acid produced by the fer- 
mentation of the surrounding organic matter ; this carbonic 
acid converts the basic lead acetate into white lead. 

The German or Austrian method is to be regarded as an 
improvement on this rough process. The heat necessary 
for the normal course of the chemical reactions is produced 
from fuel ; the carbonic acid formed by the combustion of 



78 MINERAL AND LAKE PIGMENTS. 

the fuel is used to convert the basic lead acetate into white 
lead. 

2. In the above methods the manufacture of white lead 
commences with the production of white lead from metallic 
lead and acetic acid. In the so-called French method a 
solution of basic lead acetate is decomposed by carbonic acid 
into white lead and neutral lead acetate, which latter is again 
converted into basic acetate. 

3. The English method. The principle of this process 
consists in moistening litharge with a solution of lead acetate 
and exposing it to the action of carbonic acid, whereby white 
lead is formed. 

Methods for manufacturing white lead, which are often 
advanced as entirely new processes, may be always traced 
to one of the above, from which they deviate but little in 
principle, and the deviations cannot always be regarded as 
improvements. In the following detailed account of the 
manufacture of white lead we shall adhere to the classifi- 
cation just given, according to which there are three prin- 
cipal methods : (1) Manufacture of white lead from metallic 
lead, acetic acid and carbonic acid (Dutch and German 
processes) ; (2) manufacture of white lead from basic lead 
acetate (French process) ; and (3) from litharge moistened 
with lead acetate solution (English process). 

We should remark that every year "new" processes for 
the manufacture of white lead are patented. The majority 
of these will not be mentioned ; to those acquainted with the 
principles of chemistry they at once appear impracticable. 



MANUFACTURE OF WHITE LEAD FROM METALLIC LEAD. 
(a) DUTCH PROCESS. 

This essentially primitive process, when properly con- 
ducted, produces white lead of good colour and covering 



THE MANUFACTURE OF WHITE LEAD. 79 

power, which are the properties for which this pigment is 
valued. It is now seldom used, 1 because other methods give 
a product of equal colour in a shorter time. It is, however, 
of economic interest, as showing how a branch of industry 
may rise from crude beginnings to a high state of perfec- 
tion. The operations comprised in the Dutch process are 
as follows : 

(1) Casting the lead into sheets ; (2) placing these sheets 
in pots and arranging the pots in the stacks (placing the pots 
containing lead and acetic acid in the bed of manure) ; (3) 
removing the pots from the stack ; (4) separating the white 
lead formed in the pots ; (5) further purification of the 
impure white lead by grinding, washing and drying. 

1. Casting the Lead into Sheets At first sight it would 
appear unsuitable to cast lead in sheets, since this metal 
can be readily rolled into sheets of any thickness. Experi- 
ence has, however, shown that rolled lead is only slowly 
attacked by acetic acid vapours, whilst cast sheets are rapidly 
attacked. 

An iron pan about one metre in diameter, with an iron 
cover furnished with a pipe opening into a flue, is used 
for melting the lead. This arrangement is designed to 
protect the workman from the dangerous vapours evolved 
from the molten metal. At the workman's side of this cover 
is a counterpoised slide, which only remains open when the 
counterpoise is held in check by a lever. In front of the 
kettle there is an iron plate movable about a horizontal 
axis. The lead being heated to just above its melting point, 
the workman takes 7 to 8 kilogrammes of metal' in [a ladle, 
and pours it on the plate, which is horizontal. The lead 
solidifies in a very short time, but before it 'is completely 
solid the plate is inclined towards the pan so that the still 

1 In England the Dutch process is in general use. TRANSLATOR. 



80 MINERAL AND LAKE PIGMENTS. 

liquid lead runs back into it, leaving a very thin sheet on- 
the plate. The hard sheet is removed from the plate, and 
the latter cooled by cold water to be ready for a new casting. 
The sheets made in this way are not more than 1 to 2' 
millimetres thick. They are then cut into strips of a width 
to suit the size of the pots in which they are converted 
into white lead ; the width of the sheets is generally 5 to 
6 centimetres. Since the rate at which the white lead is 
formed depends on the surface of the metallic lead, instead 
of continuous sheets the lead is generally cast into gratings. 
For this purpose an iron plate, upon which are intersecting 
strips, is used instead of the flat plate. Plates for the cast- 




FIG. 4. 

ing are also used containing grooves intersecting at right 
angles. In the first case, plates are obtained in which are 
openings meeting at right angles, and in the second case, 
according to the distance of the grooves apart, a more or less 
wide-meshed lattice work. 

2. Building up the Stacks The rolled-up lead plates 
are placed in the pots. These (Fig. 4) are somewhat conical 
in shape ; they have at some distance from the bottom a 
projecting ring, or sometimes three projections only, upon 
which the lead spiral rests. Before the spirals are put in 
position, a quantity of ordinary vinegar, about a quarter of 
a litre, is poured in. There must be sufficient room below 
the spiral so that it shall not be in contact with the vinegar. 



THE MANUFACTUBE OF WHITE LEAD. 81 

The insides of the pots are glazed at least half-way up, so 
that the liquid does not penetrate the porous earthenware. 

The pots have a capacity of about 1 litre, and a diameter 
at the top of 10 centimetres. If lead plates are used, 
the pots are about 20 centimetres high ; if gratings are used 
the pots may be lower, by which there is economy in room 
and a larger number can be placed in- one stack. 

The stacks, built up of pots and manure, are of different 
sizes ; it is not advisable to make them too small, or the loss 
of heat would be considerable. A stack 4 to 5 metres long, 
3| metres wide and 6 to 7 metres high will contain 6,000 
to 8,000 pots and 9,000 to 11,000 kilogrammes of lead. 

The stack consists of a rectangular pit walled on three 
sides ; the fourth side is open, with the earth dug out in the 
form of an inclined plane, in order to permit the introduction 
of the pots and the manure. The construction of the stack 
is commenced by placing the pots at the bottom in rows, 
avoiding interspaces as much as possible. In a stack of the 
size mentioned a layer contains 1,000 to 1,200 pots. Between 
the pots containing lead and acetic acid are arranged a 
number of larger ones containing acid only ; the object of 
these is to furnish acetic acid vapour. When the pots are 
in place, 3 or 4 lead plates are placed on each spiral, the 
top plate forming the cover ; immediately over the pots 
strong wooden planks are laid, and upon them a layer of 
boards, which must fit so tightly that nothing can fall 
through. On the boards is spread out a layer of fresh 
stable manure, with which the space between the outside 
row of pots and the wall is also filled. The layer of manure 
is 30 to 40 centimetres thick. 

Upon the lowest layer of pots, a second, third and so on 
are built up exactly in the same manner, so that the whole 
stack is filled with alternating layers of pots and manure. 

In order to prevent the cooling of the uppermost layer of 

6 



Of TMf 

VNJYER8ITY 



82 MINERAL AND LAKE PIGMENTS. 

pots, it is covered with a thicker layer of manure 60 to 70 
centimetres thick. When lead plates and the taller po-ts 
are used, a stack will generally contain 15 layers ; but when 
gratings and the smaller pots are employed, 18 layers can 
be packed into the same space. In arranging the layers of 
pots, care should be taken to leave spaces at tolerably equal 
distances, so that the air necessary for the oxidation of the 
lead may enter. To prevent the cooling of the stack at the 
front, where it is not protected by masonry, when full it is 
walled up with boards ; a board roof also protects the erection 
from rain. 

In place of manure, spent tanners' bark can be used, 
which in the same manner ferments, producing heat and 
carbonic acid. In places in the neighbourhood of large 
tanneries this spent bark is generally obtainable at lower 
prices than stable manure, which is more valuable for agri- 
cultural purposes ; the former has also a very considerable 
advantage, white lead made by means of spent tanners' 
bark being generally of a purer white than that made with 
manure. The reason for this is that in the decomposi- 
tion of animal excrement small quantities of sulphuretted 
hydrogen are produced, a gas which produces black lead 
sulphide when it comes into contact with lead compounds. 

According to the results of practical experience, pigs' 
dung cannot be used in the manufacture of white lead ; 
so much sulphuretted hydrogen is evolved from it that the 
w T hite lead is not white, but has a greyish tinge. 

When bark is used in place of manure, a discolouration 
of white lead by sulphuretted hydrogen is not to be feared, 
but there is the drawback that a longer time is necessary for 
the corrosion of the lead, because the bark decomposes 
more slowly than the manure, and accordingly gives out 
less heat and carbonic acid. 

According to the climate of the country, the stacks may 



THE MANUFACTUBE OF WHITE LEAD. 83 

be differently erected. In colder countries it is necessary to 
sink them in the earth and surround them with masonry, 
as directed above ; but in warmer climates such effectual 
protection against cooling is unnecessary, yet in all cases 
it is better to sink the stack in the earth on account of the 
regularity of temperature so obtained. 

Instead of sinking the stacks in the earth, they may be 
built in the open when there is a plentiful supply of manure 
or bark ; but they must then be surrounded by a very thick 
layer of manure to prevent cooling. It is a desirable altera- 
tion in the construction of the stacks to provide the pots 
with lids, and so avoid the use of the layer of boards 
separating each two layers of pots. The object of the 
cover is simply to prevent dirt from falling into the pot. 
It should not fit tightly on the edge, or the entry of car- 
bonic acid into the interior of the pot would be made 
difficult. The lids are therefore rounded and fit loosely 
on the pot. When pots with lids are used, the lowest 
layer is covered with manure in the ordinary way ; upon 
this again comes a layer of pots, and so on. 

The transformation of the lead may be regarded as com- 
plete in four to six weeks when manure is used, but with 
bark the time extends to ten weeks. The quantity of white 
lead obtained varies in different cases ; for example, from a 
stack 5 metres long, 4 metres wide, and 6 metres high, "into 
which 12,000 kilogrammes of lead were put, 10,000 kilo- 
grammes of white lead were obtained, and 4,000 kilogrammes 
of lead remained unaltered. In another case, for a stack of 
8 layers 280 kilogrammes of vinegar and 9,600 to 12,000 
kilogrammes of lead were used, and there was a residue of 
10 to 15 per cent, of lead. 

3. Removal and Grinding of the White Lead. The stack is 
pulled down after the lapse of the necessary time ; the lead 
plates and rolls are collected in wooden boxes and brought 



84 MINERAL AND LAKE PIGMENTS. 

into the room where the white lead is separated from the 
metallic lead. Formerly the white lead was removed from 
the sheets exclusively by manual labour, an operation 
extremely dangerous to the workman. It is quite impossible 
to prevent the formation of white lead dust, so that the 
men were continuously in an atmosphere charged with the 
poisonous material, and, as a consequence, suffered from 
the various forms of lead poisoning. 

In order to diminish as much as possible the injurious 
effects of white lead on the health of the workmen, manual 
labour has been, as far as possible, replaced by machinery, 
yet the greatest care is necessary in the different manipula- 
tions of so poisonous a substance as white lead. 

When the white lead is removed by manual labour, an 
operation which ought to be forbidden, the lead sheets are 
unrolled and struck together, whereupon the greater part of 
the white lead falls off. To remove the remainder of the 
white lead the plates are laid one upon the other and struck 
with a hammer until the white lead is loosened ; or the plates 
are cleaned with metal brushes. 

The masses of white lead obtained in this way are con- 
taminated by larger or smaller quantities of metallic lead, 
from which they must be freed by a further mechanical 
operation, namely grinding. The larger pieces of the white 
lead, which have a thickness of several millimetres, were 
picked out and sold separately under the name of flake 
white. This was formerly a highly prized quality of white 
lead, its appearance being a guarantee of its purity. The 
flake white generally found in the market nowadays is not 
obtained in this manner, but by mixing white lead with 
a solution of dextrin, forming plates from the paste and 
drying them slowly in the air. 

In order to separate the white lead mechanically from 
the remains of the lead sheets, grooved rollers are used, 



THE MANUFACTURE OF WHITE LEAD. 



85 



between which the sheets are passed. In order to guard 
against dust, the rollers are surrounded by a closed casing, 
in which there is also a sieve which serves to separate the 
larger pieces of white lead (flake white) from the fine dust. 
The arrangement is represented in Fig. 5. The unrolled 
plates pass through the opening, B, on to an endless leather 
band, by means of which they are carried between the 
grooved rollers, D and E ; after they have passed through 
these they go beween a second pair of rollers, F and G, 
which are placed nearer together ; they then fall into the 




FIG. 5. 



drum-shaped sieve, H, out of which they leave the apparatus 
in the direction of K. The w r hite lead falls through the 
sieve, is caught in trucks placed at J, and carried away to 
the mills. 

A mechanical arrangement for the separation of white 
lead from the unaltered metallic lead, due to Horn, is re- 
presented in Figs. 6 and 7. It consists of a drum, in which 
is the spindle, b, provided with the arms, c. The teeth, e, on 
the arms reach nearly to the lower curve of the drum, but 
pass at a somewhat greater distance from the upper portion 



86 



MINEBAL AND LAKE PIGMENTS. 



so that the pieces of lead can fall down again after they are 
carried up. The material is fed in through the hopper, t, 
the rotation of the arms loosens the white lead from the 
pieces of metallic lead, and it is carried on by the water which 
passes through the sieve, 3, and the stop cock, h, into the 
settling tank. The lead pieces which are carried forward 
have their progress checked by the bridge, o, they are col- 
lected by the perforated scoop, , are raised above the hopper, 
m, and fall out of the apparatus at r. If it is required to con- 
tinue the treatment of the material in the drum for a longer 



IE 




raJUjWEL 
jrfiSTg-ld^l^oj- 

".yDpM 

~mfe* iFg 




FIG. 6. 



FIG. 7. 



period it is only necessary to close the hopper, m, by the 
slide, u. 

White Lead Mills. Before the white lead is subjected to 
the real process of grinding, it is generally first ground dry, 
or, more properly, pressed or crushed. This crushing is 
accomplished by means of vertical or edge-runner mills, 
which consist of mill-stones running round upon a stone bed 
about a vertical axis. The mill is surrounded by a wooden 
casing to prevent the escape of dust. 

Wet grinding, which is done between mill-stones, may be 
carried out in two ways, with the production of hard or soft 



THE MANUFACTURE OF WHITE LEAD. 



87 



white lead. The former is obtained when the lead acetate is 
not removed, the latter when all the lead acetate is washed 
out. 

Hard White Lead presents a shining mass, broken with 
difficulty. This appearance is a guarantee against adultera- 
tion with barytes ; white lead, which contains this adulterant, 




FIG. 



does not give a smooth, but an uneven earthy fracture. 
Hard white lead is rather difficult to grind, and requires 
very careful treatment to be brought into that state of fine 
division in which it is usable. 

The dry white lead, after powdering under the edge- 
runners, must pass through a sieve, which retains the par- 



88 



MINERAL AND LAKE PIGMENTS. 



tides of metallic lead, before it is subjected to wet grinding. 
The mills used for wet grinding differ little, or not at all, 
from the ordinary pattern. 




A mill designed by Eichter of Konigsee, in Thuringia, for 
wet grinding, is constructed as follows (Figs. 8 and 9). The 
shaft, FE, is driven by a water-wheel and the necessary 



THE MANUFACTURE OF WHITE LEAD. 89 

gearing at 60 revolutions per minute ; the bevelled cogwheel 
Gr, by means of the cogwheels H and /, drives the shafts K 
and L, which, by means of the bevelled cogwheels a and 6, 
communicate to the runner stones of the mills, M, N and 0, 
a speed of 140 revolutions per minute. C D is the frame- 
work for the support of the storied arrangement of mills,/ 
and g are bearings with cast-iron cups to receive the escaping 
oil, i is a clamp fastening the axle d to k. A steel cup in k 
rests upon the hardened top of the spindle, I ; the bed stone 
rests upon the beam n (80 millimetres thick) and is prevented 
from lateral movement by the surrounding oo ; p is a wooden 
support in the opening in the bed stone, carrying the stuffing 
box, q, which prevents the white lead from running out, and 
connects the spindle, I, with the bed stone. The runner- 
stone, r, is surrounded by the box, s, cemented with white 
clay inside and out and fastened to the bed stone. The 
lever, t, turning about w, moved by the screw, u, and handle, 
v, regulates the position of the runner. The diameter of the 
stones is 95 centimetres. 

Thirty kilogrammes of w r hite lead, mixed with water, are 
fed into the top mill of the series shown on the right ; the 
mixture flows by the spouts, e, into the lower mills, and is 
received in vessels, from which it is emptied into the holder, 
P. From P it goes through the 3 mills on the left. After 
passing through 6 mills, white lead without barytes is ready 
for use. If the white lead contains an admixture of barytes, 
it is put through the 3 mills on the left a second time. In 
24 hours 900 to 1,200 kilogrammes (18 to 24 cwt.) of white 
lead are ground. 

In dry grinding, mills are used arranged so that the forma- 
tion of dust is avoided, a matter which is of particular im- 
portance. The usual size of these mills has a diameter of 
90 to 95 centimetres for both runner and bed stone. The 
grinding surfaces of both stones have radial grooves. 



90 



MINEKAL AN-D LAKE PIGMENTS. 



Fig. 10 represents the construction of Lefebre's white- 
lead mill, which is designed to give the greatest protection 
possible to the workmen. At A. the white lead masses are 
enclosed in a hopper lined with bronze, with internal angular 
projections ; by means of M, which is grooved in a similar 
manner, the larger lumps are broken up and enter the mill 




FIG. 10. 

itself by means of the hopper under A. The mill consists 
of bed stone, H, and runner, K. The grinding surfaces are 
grooved to facilitate the delivery of the ground white lead. 
As the illustration shows, the stones are completely covered 
by M, so that the escape of dust is almost completely pre- 
vented. The ground white lead is conducted by the tubes,. 
O O, to the drums, 0' 0', from which it is packed. 



THE MANUFACTURE OF WHITE LEAD. 91 

In order to obtain hard white lead, the material which 
has been ground dry under the edge runners is mixed with 
water to a soft paste, or if water has been added whilst the 
lead was under the edge stones, more is now added to make 
the paste thin enough to be ready for the mills, where it is, 
ground, being fed in regularly by means of a copper spoon. 
The paste issuing from the mill is collected in earthenware 
pans or plaster of Paris moulds, in which it is dried. 

These pans have usually the shape of a truncated cone, 
which was the form in which white lead was formerly brought 
into trade from Holland. In drying, water is lost, and the 
mass of white lead shrinks, so that the lump can be removed 
from the mould after a few days on turning it upside down. 
The drying is accomplished either in the air or in artificially 
heated stoves. The heating must at first be gradual, or the 
mass of white lead would shrink so rapidly that the cone 
would be full of cracks, and then easily fall to pieces. When 
once the drying has reached a certain stage, the temperature 
of the stove may be raised to 50 C. (12'2 F.) without danger 
of breaking the lumps. When quite dry, the surface of the 
white lead, which is now rough, must be smoothed by scrap- 
ing, when it is ready for the market. 

Soft White Lead. The hard white lead prepared as just 
described consists of very heavy and very hard lumps of the 
purest white. When soft white lead is required, the admixed 
lead acetate must be removed by washing. This is accom- 
plished by adding a larger quantity of water, either when 
grinding under the edge runners or in the mills, so that a 
thin pulp is formed ; this is run into a receiver, in which 
is a stirrer. The white lead is not completely prevented 
from sinking by the stirrer ; a soft mud is deposited at the 
bottom of the vessel, which can only be stirred up with 
difficulty on account of its high density. When this vessel 
is full, the stirrer is stopped and the milky liquid allowed 



92 MINEEAL AND LAKE PIGMENTS. 

to settle, which happens in a short time on account of the 
high specific gravity of white lead. The clear liquid above 
the white lead is drawn off into a tank lined with cement. 
It is advisable to arrange the stirrer so that it may be placed 
at any height in the vessel ; if this is the case, by gradually 
lowering the stirrer whilst in motion, the white lead lying 
at the bottom can be mixed up with fresh water. These 
operations are repeated until all the lead acetate is removed. 

Soda solution is added to the wash waters in order to 
recover the lead dissolved in them as lead acetate ; lead car- 
bonate is precipitated and settles at the bottom of the tank. 
This precipitation may also be effected by putting lumps of 
limestone in the tank. The paste remaining in the washing 
tubs, which now contains only pure white lead and water, is 
filled into bags of closely woven material and the water 
pressed out by a gradually increasing pressure, until a stiff 
pulp remains behind. This is then completely dried, either 
in the air or in drying stoves. 

Soft white lead forms either irregular lumps or a soft, 
heavy powder. The lower qualities of white lead contain 
a smaller or larger quantity of finely ground barytes ; the 
higher the proportion of barytes the smaller is the covering 
power of the mixture. A very simple method for detecting 
barytes in white lead will be given later. 

(b) GERMAN PROCESS. 

The German or Austrian method of making white lead 
is also known as the chamber process, since the formation 
takes place in closed chambers, constructed of wood or 
masonry. In the older processes, cast lead sheets were bent 
double and hung on cross bars in a wooden box with a water- 
tight bottom, with the precaution that the plates did not 
touch. A number of these boxes, generally 90, were 
arranged in a hot room, each box being about 1 P 6 metre 



THE MANUFACTUKE OF WHITE LEAD. 93 

long, 0'4 metre wide and 0'3 metre high. On the bottom of 
each box was poured a mixture of vinegar or dilute acetic acid 
and wine refuse, and the box was covered with a well-fitting 
lid. The temperature of the room was gradually raised week 
by week ; during the first week remaining at 25 C., during 
the second at 38 C., the third at 45 C. At the commence- 
ment of the fourth week the temperature was raised to 50 
C., at which it was kept for a fortnight. At this high 
temperature a considerable quantity of acetic acid is evapor- 
ated, causing the formation of lead acetate, which is con- 
verted, by the carbonic acid evolved from the wine refuse, 
into basic lead carbonate. When the proper temperatures 
have been maintained, on opening the boxes almost all the 
lead is found changed into white lead, which is knocked off, 
and the residual lead used in casting new plates. It is easy 
to conceive that the boxes may be well replaced by brick 
chambers, in which a large number of lead plates are brought 
and into which acetic acid vapours and carbonic acid are 
introduced after the room has been closed. Chambers are 
used which can contain 12,000 to 12,500 kilogrammes of 
lead. 

These chambers, in which the lead plates are hung upon 
wooden supports, have an opening immediately above the 
bottom, which is connected with a retort in which vinegar, 
containing 4 to 5 per cent, of acetic acid, is boiled. After 
about 12 hours, by the simultaneous action of the vapours 
of acetic acid and the oxygen of the air, lead acetate is 
formed. Carbonic acid is now led into the chambers. The 
carbonic acid is obtained by burning charcoal in a cylindrical 
furnace ; it is cooled by being passed through a long iron 
tube before it enters the chamber. For 12,500 kilogrammes 
of lead there are required every day about 482 litres of dilute 
acetic acid, obtained by mixing strong acetic acid with water 
until the mixture contains 4' 5 per cent, of the acid, and 18 



'94 MINEEAL AND LAKE PIGMENTS. 

kilogrammes of charcoal, from which the carbonic acid is 
obtained. The time required is 5 or 6 weeks, and the residue 
of unaltered lead varies from 10 to 35 per cent. 

The white lead obtained by this process is quite usable, 
but the process has the considerable disadvantage that there 
is no control over the quantities of materials used. In order 
to produce a certain quantity of white lead of a certain com- 
position, definite quantities of lead, oxygen, acetic acid and 
^carbonic acid are necessary. If, then, the apparatus can be 
so arranged that the quantities of material employed can be 
-accurately measured, a great advance will have been made, 
for the operation will be no longer conducted at random, but 
under definite unchangeable conditions. The quantities of 
carbonic acid and acetic acid can be calculated beforehand. 
The volumes of the gases required may be measured with- 
out difficulty by meters of the type used for measuring 
coal gas. Upon this principle are based a number of more 
modern methods. 

According to the process of Major, the vapours of water and 
acetic acid are introduced at the same time into the chambers 
filled with lead plates, in order to produce basic lead acetate. 
This part of the process requires about 12 hours. Then 
carbonic acid is introduced: it is produced by burning char- 
coal in an iron cylinder, through which air is forced. By 
means of this carbonic acid, which is at a comparatively 
high temperature, about 60 C., the lead acetate is quickly 
'changed into white lead. A portion of the lead acetate 
remains undecomposed ; in order to remove this, at the close 
of the operation ammonia is injected, which decomposes the 
lead salt. The ammonia salts now present are finally driven 
out by means of superheated steam. Major's apparatus is 
depicted in Fig. 11. A and B are chambers with horizontal 
gratings upon which the lead lies ; C is the furnace for burn- 
ing the charcoal, provided with a fan. The products of 



THE MANUFACTURE OF WHITE LEAD. 



95 



<combustion from C pass under the boiler D, their passage 
feeing regulated by the valve, a ; they then heat the boiler E, 




'containing acetic acid, and "enter the chambers, A and B, 
through the flue, 6, or are directed by the valve in b into the 
-chimney, d. The gases, after passing through the chamber, 



96 



MINERAL AND LAKE PIGMENTS. 



find an exit at e. The steam pipe,/, conducts steam into the 
boiler E, through g, and also into the chambers, A and B, thus 
heating them and providing the necessary moisture ; h is the 
funnel for filling E ; the pipe, k, carries the acetic acid vapours 
into the chambers. After the thin lead plates have been 
brought into the chambers they are closed and the temperature 
raised to 49 to 60 C., steam and acetic acid vapours are led 
in for 10 to 20 hours in order to form the basic acetate, then 
the furnace gases are introduced at a temperature of 60 C. 
The white lead obtained in this manner can be finished in 
the ordinary way by washing and grinding, but it is better to 





<g( - -NHHmrw ~ 



tZJ 



FIG. 12. 



FIG. 13. 



remove the lead acetate by introducing ammonia, and then 
not air or superheated steam, as previously stated. 

The process is complete in 2 to 4 weeks. Gartner, 
working according to this process with 150 kilogrammes of 
lead in a chamber 1'26 metre long, 0'78 metre high and 
0*78 metre wide, obtained good white lead in 28 days. 

The apparatus designed by H. Kirberg for the manu- 
facture of white lead is illustrated in Figs. 12, 13 and 14. 
The lead plates are hung upon the laths, a, in the chambers ; 
the supports of the laths, b, go through the slits, c, in the sup- 
ports, d, and project through the walls at e. The carriers, b^ 



THE MANUFACTURE OF WHITE LEAD. 



97 



hang from bolts by brass wire. By striking the end of the 
laths, e, they are made to swing so that the white lead loosely 
adhering to the lead plates is shaken off and fresh surfaces 
of metal are exposed. The openings in the walls through 
which the ends of the laths project are closed by indiarubber. 
In order to prevent the production of dust when the chambers 
are emptied, water is introduced in a fine spray upon the lead 
plates from the copper tubes, v, placed under the roof of the 
chamber. The water washes off the remains of the white 
lead from the plates. 




FIG. 14. 

In a similar manner, the white lead process may be 
carried out in a shorter time when the gases enter the 
chamber under increased pressure ; but this is attended with 
difficulties, since continuous supervision of the apparatus 
and of the tightness of the chamber is necessary. 

The apparatus of W. Thompson (Fig. 15) consists of a 
chamber, A, constructed with a false roof, B, to carry off 

the condensed vapours, and provided at both sides with 

7 



98 



MINERAL AND LAKE PIGMENTS. 



doors, through which the waggons, carrying the lead, are 
introduced and removed. The pipes, n, which convey air 
and carbonic acid, are provided with branches, e, which reach 
to the sides of the chamber, and are joined to the perforated 
tubes, D. The pipes, n, are provided with reservoirs, b, which 
prevent the too rapid entry of the air or carbonic acid, and 
at the same time give a sufficient heating surface. Upon 
the waggon, E, are the frames, v, which support the lead 
plates. The plates may be solid or gratings ; they are 3 to 12 




FIG. 15. 

millimetres thick, and are arranged at intervals of about 25 
millimetres in the frames. The troughs, F, for the reception 
of the acetic acid are filled from the reservoir, Z '. The steam 
pipe, C, effects the evaporation of the acid ; the steam pipe, d, 
heats the chamber at the commencement of the process, and 
also when C is out of use. When the chamber is filled with 
lead, the troughs, F, are filled with acetic acid of 5 per cent, 
strength. The chambers are closed, and steam is led 
through C and d until the temperature in the chamber 



THE MANUFACTURE OF WHITE LEAD. 99 

reaches 25 to 50 C., and the evaporation of the acid begins. 
Then air is forced in under a small pressure through 
n, e, D during 3 to 4 days, after which follows a mixture of 
equal parts of carbonic acid and air, continued until the 
formation of white lead is complete. When plates 3 milli- 
metres thick are used, and the temperature is gradually 
raised from 25 to 50 C., about 12 days are required ; but 
if the plates are 12 millimetres thick, 28 days are necessary. 
For the proper carrying out of this process it is important 
that the temperature of the chamber should be very gradually 
raised from 25 to 50 C. 

In the process of P. Key molten lead is poured in a thin 
stream into water, and the " granulated " metal then placed 
in vessels, in a layer 30 centimetres thick, upon a grating 
5 centimetres above the bottom. In the bottom are narrow 
tubes which, reaching above the lead, serve to admit air. 
Acetic acid is allowed to flow over the lead from vessel to 
vessel. In order to obtain the proper solution, a layer of 
lead, 2 metres thick, is necessary, so that 6 to 7 vessels are 
arranged, one above the other. If lead acetate solution be 
used, the layer of lead need only be 1'2 metre thick. The 
solution of basic lead acetate is then treated with carbonic 
acid according to the ordinary process. 

In addition to the modifications of the German white 
lead process which have been described, many others have 
been proposed. The principle of many of these methods is, 
that finely divided lead is much more rapidly converted into 
white lead than lead in the form of plates ; finely divided 
lead exposes an enormously greater surface to the action 
of the acetic acid and other materials than do lead plates. 

In Rostaing's method the lead is changed into very small 
pellets, by allowing the melted metal to flow on to a rapidly 
rotating iron disc ; the molten lead, in consequence of the 
centrifugal force due to the rapid rotation, assumes the form 



100 MINERAL AND LAKE PIGMENTS. 

of very small drops, which are thrown off the disc and cooled 
in a vessel of cold water. 

Torassa recommends that lead, obtained in pellets by 
pouring into cold water, should be brought into a rapidly 
rotating vessel, whereby the greater part will be converted 
into fine dust (?). This lead dust is said to be then con- 
verted into white lead by simple exposure to air, lead oxide 
being first formed, and from it basic lead carbonate. This 
process is not workable on a large scale. The rapid oxida- 
tion of finely divided lead by air has been applied by several 
inventors to the manufacture of white lead. The processes 
of Woods, M'Cannel and Griineberg are founded upon this 
transformation. In essentials they are as follows : lead is 
finely divided in iron or earthenware cylinders, and either 
carbonic acid and air, or a mixture of these with acetic 
acid vapour, are introduced by means of the axle of the 
cylinders. 

If a special apparatus is not provided, as it should be, on 
the large scale, to separate the unaltered lead from the white 
lead, the lumps coming from the chambers must be sub- 
jected to a process of levigation, in which the lead, being the 
heaviest body, will be first deposited ; the last portions de- 
posited consist of the purest white lead, the lower layers of 
which will be tinged more or less grey by an admixture of 
finely divided lead and lead peroxide. 

White lead manufactured by the German process occa- 
sionally exhibits perceptible reddish or greyish tinges, the 
cause of which lies in the defective execution of the process. 
The red tinge denotes the presence of free lead oxide, caused 
by the use of an insufficient quantity of acetic acid. A grey 
tint is due to the presence of metallic lead or an excess of 
lead carbonate. 



THE MANUFACTURE OF WHITE LEAD. 101 

(c) FRENCH PROCESS. 

The French process for making white lead is based upon 
the reaction which occurs when carbonic acid is passed into 
a solution of basic lead acetate ; basic lead carbonate is preci- 
pitated and neutral lead acetate remains dissolved ; the latter 
is again converted into basic acetate, from which carbonic 
acid again separates white lead and so on. This process, at 
present used on an enormous scale, is due to the French 
chemist Thenard, who first put it into operation on the large 
scale at Clichy, near Paris. The method is also known as 
the Clichy process. 

The operations of this process are divided into the pro- 
duction of the basic acetate and the treatment of its solu- 
tion with pure carbonic acid, whereby the basic carbonate is 
precipitated. 

1. Preparation of the Solution of Basic Lead Acetate 
The preparation of this compound has been already described ; 
the following is supplementary to what was previously given. 
If litharge be used, it is dissolved in wooden tubs heated by 
steam. The acetic acid is brought nearly to boiling by open 
steam, and the finely ground litharge gradually added. In 
consequence of its high specific gravity, the litharge would 
quickly sink to the bottom, so that it is advisable to keep the 
liquid in motion by means of a stirrer, and to allow the 
litharge to fall in in a thin stream. The introduction of the 
latter is continued until the specific gravity of the solution 
indicates that the liquid contains three equivalents of lead 
oxide to one equivalent of acetic acid. 

In working with metallic lead, this must be used in a 
finely divided form ; it is cast, as in the Dutch process, into 
thin sheets or gratings, or into flat wires or ribbons. These 
ribbons are easily made by melting the lead in a pan provided 
with a delivery pipe with stop cock ; beneath the latter is 



102 MINERAL AND LAKE PIGMENTS. 

brought a vessel, which can be moved backwards and for- 
wards upon a tram line, filled with water. When the melted 
lead is allowed to flow into this vessel, which is being moved 
backwards and forwards, the metal forms long, thin ribbons, 
which possess a large surface. A wooden tub is almost com- 
pletely filled with these lead ribbons, which are then covered 
with acetic acid. After a short time the acid is run off, 
when, by the action of the air, so energetic an oxidation of 
the lead takes place that the contents of the tub become 
heated, and steam and acetic acid vapours begin to rise. 
When this is seen, the original acetic acid is pumped back 
into the tub, and left there for some hours in contact with 
the lead in order that it may dissolve the lead oxide. When 
the solution has reached a specific gravity of 113:26 to 1*1415, 
it is drawn off from the undissolved lead, which again, in a 
short time, in consequence of the rapid oxidation, becomes 
warm, and is treated with fresh acetic acid. 

The lead ribbons become finally so thin that they fall 
together by their own weight, and form tight masses, upon 
which air and acetic acid can no longer act. These residues 
are then removed from the solution vessel and new ribbons 
introduced. The residues have a velvety appearance ; when 
they are mixed up with water they make it dark, and 
from the turbid liquid a fine, velvet-black powder soon 
separates, which consists of finely divided silver. The liquid 
still remains turbid owing to the 'suspension of fine particles 
of carbon. The lead ores, especially galena, often contain 
notable quantities of silver ; silver lead is generally desilver- 
ised before use ; usually, however, small quantities of silver 
remain in the lead. When the lead is dissolved in acetic 
acid, this silver settles as a soft powder at the bottom of the 
vessels in which the lead residues from the solution tubs are 
washed. 

'2. Preparation of the Carbonic Acid and Precipitation of 



THE MANUFACTUEE OF WHITE LEAD. 



103 



the White Lead. The carbonic acid required for precipi- 
tating the basic carbonate is obtained either by heating 
limestone in a small furnace, from which the gas is drawn 
by a pump, or directly by burning charcoal. In the former 
case very pure carbonic acid is obtained, and, as a by-product, 
valuable quicklime ; in the latter case precautions must be 
taken to produce pure carbonic acid. 

The furnace designed by Kindler for preparing carbonic 
acid (Fig. 16) consists of a conical furnace, burning coal or 
coke upon the hearth, a. The passage, c, is divided by a 
vertical wall, in order to avoid obstruction from the piling 




FIG. 16. 

up of fuel. The space, K, is filled with limestone, through 
which the carbonic acid passes ; the tanks, e, filled with 
water, cool the limestone, and the gas, which then passes 
through the water in the washing vessel, D, is drawn off 
by a pump. 

In the old process at Clichy the apparatus depicted in 
Fig. 17 was used. The basic lead acetate was made in the 
wooden tub, A, provided with the stirrer, B C. The solution 
was run off from this vessel by means of the cock, b, into 
the settling tank, E, in which the mechanical impurities 
separated from the solution. The clear liquid ran into the 



104 



MINERAL AND LAKE PIGMENTS. 



decomposing vessel, a large shallow covered tank, holding 
9,000 to 10,000 litres. In this tank opened 800 copper tubes, 
given off from the large pipe, S. The small furnace, D, in 
which limestone was burnt with coke, produced the carbonic 
acid ; from the pipe at the top of the furnace the carbonic 
acid was brought into the Archimedean screw, h K, was 
washed with water, and pumped into the solution of lead 
acetate. The introduction of the carbonic acid was con- 
tinued from 10 to 12 hours, after which the apparatus was 
left at rest until the liquid in the decomposing tank had 
become quite clear, through deposition of the white lead. 
The clear solution, now containing neutral lead acetate, was 




FIG. 17. 

run off into the receiver, in, from which the pump, P, carried 
it back into the dissolving tub, A, where it was treated with 
fresh quantities of litharge. The solution of neutral lead 
acetate drawn off from the white lead had approximately 
a specific gravity of 1*0901. The white lead at the bottom 
of the decomposing tank was a tolerably thick paste. It 
was transferred to the tank, 0, and washed several times 
with water. The first wash waters, which contained small 
quantities of lead acetate, were returned to the dissolving 
tub. The resulting white lead formed a very soft powder ; 
it was at once placed in the drying pans. The white lead 
prepared by this process is a precipitate containing no coarse 
lumps, so that grinding is unnecessary. 



THE MANUFACTURE OF WHITE LEAD. 



105 



Theoretically, the quantity of acetic acid with which the 
process is commenced is sufficient to form an unlimited 
quantity of white lead, since all the acetic acid brought 
into the decomposing tank in the form of basic acetate is 
returned to the dissolving tub as neutral acetate. In practice, 
however, matters are somewhat different. Small quantities 
of acetic acid are lost in the wash waters ; each time the 
solution of lead acetate is pumped back into the dissolving 
tub, a small quantity of acetic acid must be added to make 
up the loss. 




FIG. 18. 

The method pursued by Ozouf, in France, is a consider- 
able improvement on Thenard's process. Pure carbonic acid 
is used for the precipitation, and white lead of similar com- 
position to that produced by the Dutch process is obtained, 
since the introduction of the carbonic acid can be regulated 
according to the volume and strength of the lead solution, and 
thus white lead of any desired composition can be produced. 
The most elaborate precautions for the health of the work- 
people are taken. 

The preparation of pure carbonic acid gas is based upon 



106 MINERAL AND LAKE PIGMENTS. 

the absorption of this gas from a mixture of gases by a- 
solution of sodium carbonate, and its evolution on heating 
the solution. The apparatus is shown in Figs. 18 and 19. 
The products of combustion obtained from the stove, A, are- 
drawn by the air pump, E, through the pipe, C, into the 
cooler, B, which is regularly fed with cold water by D. 
The gases compressed in the receiver, E', deposit moisture- 
there, and then proceed through 3 horizontal cylinders,. 
F, of sheet iron, provided with agitators, in which the 
carbonic acid is absorbed by a cold solution of sodium 
carbonate of 9 B. The unabsorbed gases escape into the 
atmosphere through G (Fig. 19). The sodium bicarbonate 
solution is received in the wooden tank, H, after passing 
through the 3 cylinders, F. The pump, 7, of the alter- 
nating pump, 1 1', lifts the sodium bicarbonate solution out 
of H and sends it through the pipe, K, into the tubular 
cylinder, /, which stands upon a cylinder of larger diameter, 
J', communicating with it only by the vertical tubes. The 
bicarbonate solution rises between the tubes in /, passes 
through the pipe, L, drops in a fine spray through the rose 
forming its mouth, and by means of the vertical tubes passes 
into J' and thence into M, where it is heated by means of 
a steam coil to 100 C. Carbonic acid is then evolved, and : 
the residual solution of neutral sodium carbonate, after 
cooling in the vessel, R, by means of the cold coil, is drawn 
off by the pump, 7', again to enter the cylinders, F, by means 
of the pipe, K. The carbonic acid evolved in 37, together 
with steam, enters J' through ,Y, and in rising in the tubes 
of the cylinder, J, is cooled by the falling spray of bicarbonate 
solution. The cooling is completed in the coil, 0,. surrounded 
by water ; the vessel, P, separates the condensed water and 
passes the gas on into the holder, Q. The pipe, S, connecting 
P with the suction pipe of the purnp, J', serves to restore to 
the solution of sodium carbonate the water it has lost, thus, 



THE MANUFACTUBE OF WHITE LEAD. 107 

maintaining the proper concentration. The cost of 1 cubic 




metre of carbonic acid is 10 centimes, of 1 kilogramme 5 
centimes. 



108 MINERAL AND LAKE PIGMENTS. 

For the production of white lead, the carbonic acid, by 
means of the pipe, U (Figs. 19 and 20), enters the cylinder, T, 
provided with an agitator and containing a solution of basic 
lead acetate. By means of the pump, V, the lead solution is 
fed into the cylinder, T, through W. The absorption of the gas 
proceeds rapidly ; the progress of the operation is followed 
by the observation of a pointer moving over a scale ; as the 
gas holder sinks the pointer moves upwards. After the pre- 
cipitation of white lead, the contents of T are emptied into 
the tub, b, in which rotate rakes attached to a vertical axis 
of iron plated with copper. When the white lead has settled, 
the supernatant solution of neutral lead acetate is drawn off 
through the pipe, c, by means of the pump, d, and conducted 
into the water-tight vessel, X, containing a stirrer on the 
vertical axis, W, made of coppered steel. Here litharge is 
added, and the resulting solution of basic lead acetate is 
conveyed to the cylinders, T, by means of the pump, F, as 
already described. The white lead in the tub, 6, by putting 
the stirrer in motion, is washed once w r ith water which 
has been previously purified by a little lead acetate. It 
then goes into another tub provided with stirrers, where 
it is several times washed, sodium carbonate being added 
to the last wash water until a sample of the white lead is 
not coloured by a drop of potassium iodide solution. In this 
way the wash water is obtained free from lead, and the 
product is said to be of better quality. This, however, is 
not in accordance with the fact that good Dutch white lead 
.generally contains some lead acetate. The two-cylinder 
pump, h, which is in connection w r ith the gas holder, forces 
gas over the surface of the liquid in T in order to drive 
it into tubs which are not in the position shown for b in 
the illustration, and into which there is no direct flow. The 
washed white lead is brought into bags which are pressed in 
a hydraulic press, dried, ground, sieved and packed in casks. 



THE MANUFACTUEE OF WHITE LEAD. 109 

These troublesome and often dangerous operations have been 




modified by Ozouf in the following manner. The pulp white 
lead runs from the tub, 6, into the hopper, g, where it is kept 



110 MINERAL AND LAKE PIGMENTS. 

mixed by a small stirrer, and from which it passes on to the 
cylinder, /, heated by gas from the inside. In its rotation 
the cylinder carries along the white lead and dries it, it is 
then removed by a knife below the hopper, and falls on to 
an inclined plane. The hopper and cylinder are in a room 
provided with a good draught. 

The lumps coming from the drying room are placed by 
workmen wearing respirators in buckets on an endless chain, 
are carried to the mills, ground and sieved ; then, by means of 
an Archimedean screw, the white lead is conveyed to a cask 
in which it is evenly pressed by means of a special mechanism. 
A bell announces when a cask is full. 

Manufacture of White Lead by means of Natural Carbonic 
Acid, In districts where currents of carbonic acid gas issue 
from the ground, they can be used in the manufacture of 
white lead, and are actually utilised for this purpose. 
Natural carbonic acid may, of course, be used for any of the 
white lead processes. 

(d) ENGLISH PROCESS. 

In this process, now no longer in use, white lead was 
obtained by mixing litharge to a stiff paste with a weak 
'solution of lead acetate and exposing the paste to the action 
of carbonic acid. By continually kneading the mass by 
means of grooved rollers or of rotating cylinders, through 
the hollow axis of which carbonic acid was led, the paste 
was thoroughly brought into contact with the carbonic acid. 

By this process a good product is only obtained when 
pure litharge, entirely free from the oxides of iron and 
copper, is used. The copper oxide may be removed from 
the litharge by means of ammonia if this can be obtained 
at a low price ; but oxide of iron cannot be removed, and 
very small quantities of it are sufficient to impart a yellow 
tinge to the white lead. 



THE MANUFACTURE OF WHITE LEAD. Ill 

(e) OTHER METHODS. 

In Payen's process the lead sulphate obtained in consider- 
able quantities as a by-product in calico printing is the raw 
material employed. By treating this lead sulphate with a 
solution of ammonium or sodium carbonate, white lead and 
ammonium or sodium sulphate are produced. The white 
lead is then freed from the soluble salts by washing, mixed 
with a small quantity of lead acetate, and pressed into the 
drying moulds. 

By boiling lead sulphate with caustic soda and passing 
in carbonic acid (Puissant's process), a white lead is obtained 
which differs considerably in composition from ordinary white 
lead. 

Many methods have been proposed with the object of 
converting insoluble lead salts, obtained as by-products or 
by an inexpensive process, by treatment with alkaline or 
alkaline earth carbonates, into white lead. The fact that 
none of these methods has obtained a permanent footing in 
the industry shows that each must be accompanied by 
serious defects, or can only be practicable under peculiar 
conditions. 

Magnesium carbonate is used in Pattison's process to 
decompose lead chloride. Dolomite (magnesian limestone) 
is the raw material for the magnesium carbonate. Coarsely 
powdered, it is heated at a low red heat, when magnesia 
is formed, the calcium carbonate remaining almost entirely 
unaltered, since it requires nearly a white heat for its decom- 
position. The powder ground in water was, when treated 
with carbonic acid under a high pressure, soluble, magnesium 
bicarbonate being formed, the saturated solution of which 
contains 2*3 per cent, of magnesia, and has a specific gravity 
of r028. The solution of lead chloride contains 1 part of the 
;salt in 126 parts of water ; it is mixed with a slight excess of 



112 MINERAL AND LAKE PIGMENTS. 

the magnesium carbonate solution as quickly as possible. 
The liquid is drawn off from the mixing vessel into a large 
receiver in which a precipitate deposits, consisting of white 
lead and a little oxychloride. After drying, the precipitate 
is ground with a small quantity of caustic soda to decompose 
the oxychloride. A few days afterwards the mass is washed 
to remove sodium chloride and the product dried. 

The process of Dale and Milner is similar to the above 
magnesia process. Litharge, lead hydroxide or insoluble 
lead salts are mixed with sodium bicarbonate solution, and, 
with repeated additions of water, ground until the formation 
of white lead is completed. The lead compound and sodium 
bicarbonate are used in equivalent proportions. 

According to the process of P. Bronner (German patent 
52,262), 3 molecules of freshly-precipitated lead sulphate are 
heated with a solution of 2 molecules of caustic soda, when 
the basic sulphate 2 PbS0 4 .Pb(OH) 2 is formed according to 
the equation 

3 PbSO 4 + 2 NaOH = 2 PbS0 4 .Pb(OH) 2 + Na 2 S0 4 . 

Or 4 molecules of lead sulphate are decomposed by 2 mole 
cules of caustic soda, according to the equation 

4 PbS0 4 + 2 NaOH = 3 PbSO 4 .Pb(OH) 2 + Na^SO,. 

This transformation takes place at a temperature of 70 C. 
The resulting basic sulphate, although pure white, cannot 
be used as a pigment on account of its lack of covering 
power ; but by heating with a solution of sodium carbonate 
it is converted into white lead. 

2 PbS0 4 .Pb(OH) 2 + 2 Na 2 CO 3 - 2 PbCCX . Pb(OH) 2 + 2 Na^SO 4 . 

3 PbSO 4 .Pb(OH) 2 + 3 Na^CO;, = 3 PbCO,.Pb(OH) 2 + 3 



By this process, which is harmless to the workmen, the lead 
sulphate obtained as a by-product in the preparation of mor- 
dants for calico printing, can be converted into good saleable 



THE MANUFACTURE OF WHITE LEAD. 113 

white lead. The lead sulphate may also be obtained from 
litharge, lead acetate or nitrate. 

It occasionally happens that white lead has a rose tint, 
which is clearly perceptible by comparison with a pure white 
sample. This colouration occurs in white lead made from 
argentiferous lead. A very small quantity of silver is suf- 
ficient to produce the tinge of colour. 

Occasionally white lead which has been ground in oil and 
used for painting turns perceptibly yellow, the colouration 
being similar to that observed on a surface painted with 
white lead from which light is almost excluded. The yellow 
colouration is due to lead oxide. This has been proved by 
suspending such a white lead in water and treating it with 
carbonic acid, after which a surface painted with it remains 
permanently white. 

(f) OXYCHLORIDE WHITE LEAD. 

Under the name of white lead, but differing from it in 
composition, various products are found which consist of 
lead oxychloride. This compound is also known as Patti- 
son's white lead. 

Pattison's white lead can be much more cheaply manu- 
factured than real white lead, the raw material employed be- 
ing the cheap galena. The finely-powdered mineral is boiled 
with strong hydrochloric acid in closed lead vessels. Sul- 
phuretted hydrogen is evolved, which may be burnt to 
sulphur dioxide and so used to make sulphuric acid. A hot 
saturated solution of lead chloride remains, from which the 
salt separates in small crystals on cooling. The crystals are 
drained in a basket and washed with cold water to remove 
the acid. The pure lead chloride is then dissolved in hot 
water and mixed with lime water. Pattison obtained lime 
water from dolomite by burning it, treating with a little 

water to remove the easily soluble salts, and, after the 

8 



114 MINERAL AND LAKE PIGMENTS. 

removal of this wash water, treating the residue repeatedly 
with water in order to obtain a clear solution of pure hydrate 
of lime. When pure limestone is used, it may be treated 
with water immediately after burning without any pre- 
liminary preparation. 

Two equivalents of lead chloride are used to one equi- 
valent of calcium hydroxide. Practical experience showed 
that the best product was obtained when the precipitation 
was very rapidly brought about. With this object, both 
solutions entered the precipitation tanks through pipes 
with narrow slits at the side, so that the liquids met in 
a thin layer, in which the precipitation of the pigment 
was instantaneous. It is also necessary that lead chloride 
should be in excess throughout. The liquid is allowed 
to stand for the precipitate to settle, which it does in a 
brief time on account of its high specific gravity. The 
solution now contains the small excess of lead chloride 
in addition to calcium chloride ; lime water is added until 
the liquid turns red litmus paper blue. From the alkaline 
solution all the lead soon separates as lead hydroxide, 
which is dissolved in hydrochloric acid, and thus again 
comes into the process. 

In order to utilise the large quantities of hydrochloric 
acid obtained in the manufacture of soda, Percy described 
a process in which galena is ground with hydrochloric 
acid, whereby in 30 to 40 hours all the lead is con- 
verted into lead chloride, whilst the stony admixtures are 
unattacked. The lead chloride is then separated by levi- 
gation from the undissolved minerals and washed until 
free from iron, when it is dissolved in hot water and 
converted into oxychloride by means of lime water. 

Lead Sulphite, PbS0 3 , can be obtained by passing sul- 
phur dioxide into a solution of basic lead acetate ; lead sul- 
phite is precipitated and a solution of neutral lead acetate 



THE MANUFACTUBE OF WHITE LEAD. 115 

remains. The process is similar to the French white lead 
process, with the difference that sulphur- dioxide is used 
instead of carbon dioxide. Lead sulphite has no advantages 
over white lead, and is more expensive ; it has thus never 
found practical application. 

Lewis and Bartlett's White Lead Pigment, In the lead 
works at Zoplin, in Missouri, galena is smelted with lime- 
stone and coal, lead fume being obtained in addition to 
metallic lead. The lead fume deposits are ignited, and again 
worked for lead and lead fume. This last lead fume can at 
once be used as a white pigment ; it consists principally of 
lead sulphate, lead oxide and zinc oxide. 

WHITE LEAD-ANTIMONY PIGMENTS. 

Lead antimonite and antimonate are both heavy, white 
powders which can be used as pigments. They are dearer 
than white lead, to which they are inferior in covering 
power, and which they do not exceed in permanence. 

Lead Antimonite is obtained by heating 5 parts of finely 
powdered antimony with 20 parts of sulphuric acid until a 
dry, white mass of antimony sulphate is left. This is fused 
with soda ash, the melt is extracted with water, and lead 
antimonite obtained by precipitating with lead acetate. 

Lead Antimonate is formed by introducing in small quan- 
tities at ,a time a mixture of 1 part of finely powdered stibnite 
(antimony trisulphide) with 5 parts of sodium nitrate into a 
red-hot crucible, boiling the mass with water and precipitat- 
ing the solution with lead acetate. 



CHAPTEE VIII. 

ENAMEL WHITE. 

BARIUM sulphate, known as permanent white, enamel white, 
blanc fixe, barytes white, is the only white pigment which 
is absolutely unaltered by exposure to the atmosphere. 
Lead pigments are discoloured in the course of time, and 
in the end turn black ; bismuth white behaves in the same 
manner ; zinc white is much more lasting, but not quite 
permanent. 

. Enamel white is really permanent ; it deserves the 
greatest attention from the colour manufacturer, especially 
as it can be made by a very simple and cheap method. 
When sulphuric acid or a soluble sulphate is added to the 
solution of a barium salt, all the barium is at once precipi- 
tated in the form of barium sulphate. 

When quite pure, barium sulphate forms an extremely 
soft, brilliantly white powder, which offers complete resist- 
ance to the action of the atmosphere, and also of strong acids 
and alkalis. It is extremely insoluble, and can be precipi- 
tated from the most dilute solutions, and is then obtained 
in so fine a state of division that it cannot be filtered from 
the liquid ; it passes through the closest filter together with 
the liquid. When the barium solution is heated to boiling 
before precipitation, the precipitate is somewhat coarser, and 
can be filtered off without difficulty. 

Barium sulphate occurs ready formed in nature as the 
mineral barytes or heavy spar. Finely ground barytes may 



ENAMEL WHITE. 117 

be used alone as a pigment, but more commonly is used for 
reducing white lead, for which purpose it is particularly 
applicable on account of its high specific gravity. This 
admixture must be regarded as diminishing the quality of 
the pigment, because ground barytes has far less covering 
power than white lead. Artificial barium sulphate is in a 
state of division which cannot be reached by grinding barytes, 
consequently it considerably surpasses the latter in covering 
power. 

The raw material for the manufacture of enamel white 
is either barytes or witherite (barium carbonate) ; the latter, 
however, occurs so rarely, in comparison with barytes, that 
the greater quantity of all barium compounds is obtained 
from barytes. 

If witherite is obtainable in large quantity, enamel white 
can be prepared from it by dissolving in hydrochloric acid 
and precipitating the solution of barium chloride so obtained 
by sulphuric acid. If the witherite is very pure, the process 
may be simplified by treating the mineral directly with sul- 
phuric acid, and separating the enamel white by a process of 
levigation from the impurities. In this case it is, however, 
necessary to add a small quantity of hydrochloric acid to the 
sulphuric acid, for the -latter forms on the surface of the 
witherite, at the commencement of the reaction, a thin layer 
of barium sulphate, which is quite sufficient to prevent the 
further action of the acid on the witherite lying below. The 
hydrochloric acid forms barium chloride, which is at once 
decomposed by the sulphuric acid into barium sulphate and 
free hydrochloric acid ; this again dissolves a fresh quantity 
of witherite, and this process is repeated until the mineral is 
completely and quickly dissolved. 

Enamel white is, however, generally prepared from 
barytes, which is ground into a very fine powder and con- 
verted into barium sulphide by heating with coal (see pages 



118 MINEEAL AND LAKE PIGMENTS. 

41 and 42). Hydrochloric acid acting on the sulphide 
produces barium chloride and sulphuretted hydrogen. 

The covering power of a pigment is greater the finer its 
state of division, so that it would appear advisable to pre- 
cipitate a weak solution of barium chloride by sulphuric acid 
at the ordinary temperature. When the barium sulphate 
has been completely precipitated, a solution of pure hydro- 
chloric acid remains, which ought to be utilised ; but when 
very dilute barium chloride solution is used, the hydrochloric 
acid is so dilute as to be useless. The barium chloride is, 
therefore, given in practice such a strength that it has a 
specific gravity of about 1'198; when the barium sulphate 
has been precipitated from this solution the residual hydro- 
chloric acid has a specific gravity of 1'043. 

Water of considerable purity must be used to dissolve 
the barium chloride. Experience has shown that water 
which contains appreciable quantities of organic matter does 
not give a pure white product. The presence of sulphate of 
lime in the water, which precipitates barium sulphate, need not 
be regarded, because the barium sulphate is so finely divided 
that it remains suspended in the liquid, and is carried down 
on precipitation of the enamel white by sulphuric acid. Car- 
bonate of lime in the water causes the separation of barium 
carbonate ; this may be avoided by slightly acidifying the 
barium chloride solution, thus converting the calcium car- 
bonate into chloride. 

According to C. A. F. Meissner, artificial barytes, suitable 
for use in oil paints, is obtained by precipitating barium salts 
by soluble sulphates in place of sulphuric acid, then quickly 
heating the washed and dried precipitate in a muffle to a red 
heat and throwing into cold water. 

As has been already stated, enamel white is the most 
permanent pigment that exists ; it appears destined in 
course of time to replace white lead and all other white 



ENAMEL WHITE. 119 

pigments, especially as its cost is generally lower than that 
of the other white pigments. It costs, for example, only 
half as much as white lead. At present, the principal uses 
of enamel white are found in paper staining ; it is not used 
to any extent in oil paints. On account of its permanence, 
it should be used in the place of white lead and white zinc. 
It also appears particularly suitable for obtaining pale shades ; 
it can be mixed with any other pigment in any quantity with- 
out altering it in the least. This is, of course, only true when 
the enamel white is completely pure, and when it has been 
freed from every trace of hydrochloric acid by careful wash- 
ing. 

LITHOPONE. 

A white pigment is obtained, according to Orr's process, 
by lixiviating crude barium sulphide, obtained by igniting 
barytes with coal, with water and dividing the solution into 
two parts. Zinc chloride is added to the first portion, then 
zinc sulphate, and finally the second portion of barium sul- 
phide solution. The white precipitate obtained by this 
process contains one equivalent of barium sulphate to two 
equivalents of zinc sulphide. It is collected, quickly dried, 
heated in retorts to redness, and, whilst still hot, thrown into 
cold water, by which its density and therefore covering 
power are increased. The pigment is finally washed and 
ground. It is a good white, but when mixed with lead 
pigments discolours them by reason of the sulphide it con- 
tains. 



CHAPTEK IX. 

WASHING APPARATUS. 

IN all colour works operating on a large scale, special appar- 
atus is used, in which are carried out the washing, pressing 
and drying of the pigments obtained as precipitates. Only 
pigments prepared in small quantities are filtered through 
filter paper. The treatment of enamel white and white lead 
requires the use of apparatus for this purpose in a special 
degree. We insert a short description here. 

The preparation of enamel white takes place most con- 
veniently in tubs provided with a stirring apparatus. When 
the precipitate of barium sulphate has once settled to the 
bottom, it is very difficult to again mix it up with water by 
means of a hand stirrer, an operation which must be often 
performed in washing. If vessels be used provided with a 
suitable mechanical stirrer, the precipitates are rapidly and 
thoroughly washed. We have already stated that vessels 
with a stirrer capable of being raised out of the liquid were 
specially suitable for washing white lead. Such an arrange- 
ment can with advantage be used in washing all precipitates. 
Many of the mineral pigments have to be freed from admixed 
salts by washing. A description follows of an effective 
washing apparatus with movable stirrer. In the cylindrical 
vessel (Fig. 21), which may be of any size, is a vertical iron 
shaft rotating upon a pin in the bottom of the vessel. On 
this axis is a horizontal wooden crosspiece, the under surface 
of which is studded with brushes ; the disc, which this cross- 



WASHING APPAEATUS. 121 

piece carries, is united by means of two bars with a second, 
in which is cut a screw moving on the screwed shaft. The 
shaft is rotated by means of the cogwheels shown in the 
illustration. The handle fastened to the upper disc enables 
the crosspiece carrying the brushes to be raised or lowered. 
When the handle is held fast, this crosspiece rises or falls 
according to the direction of rotation of the axis. The pipe 
shown at the side supplies the water for washing the 
precipitates. 

In the preparation of colours liquids have often to be 




FIG. 21. 



brought into the precipitating vessel which would attack 
the iron of the stirring apparatus, so that it is advisable to 
make the connecting rods between the two discs of such 
a length that the screws may be above the vessel. All iron 
parts of the apparatus dipping into the liquid should be 
protected by asphaltum varnish. 

When a specially heavy precipitate is to be washed, 
such as enamel white, chrome yellow or white lead, the 
stirrer is raised as high as possible before the commence- 
ment of the operation. When the precipitate has formed 



122 MINEEAL AND LAKE PIGMENTS. 

and settled, the liquid is run off, the water tap opened, and 
the stirrer slowly brought down to the precipitate ; the 
brushes fastened to the crosspiece stir up first the top of 
the precipitate, then the next portions, and so on until the 
whole of the precipitate has been stirred up into the liquid. 
When this has been accomplished, the stirrer is kept going 
for some time, so that the water may take up as much of 
the soluble materials as possible ; it is then raised out of 
the vessel, in which the precipitate again settles. 

As a rule, two or three washings of permanent white, in 
an apparatus of the construction described, are sufficient 
to render it quite free from acid. Washing must be con- 
tinued until the wash water leaves blue litmus paper quite 
unchanged. When dry, precipitated barium sulphate is 
a very soft powder of great whiteness, which, on account 
of the fineness of its particles, can be readily ground with 
binding materials. 

Enamel white loses, in a remarkable manner when com- 
pletely dry, a great portion of its covering power and of its 
valuable property of being easily ground with oil or size 
to a homogeneous paste. It is not known whether this 
alteration is caused by a molecular change of the barium 
sulphate, as is not altogether improbable. In order to 
preserve the valuable properties of enamel white it is not, 
as a rule, completely dried, but is brought into the market 
in the form of pulp, which is obtained by bringing the 
washed precipitate into strong linen bags and allowing the 
water to drop through. This object is more quickly ac- 
complished when the last wash water is drawn off only 
to such a point that, when the precipitate is again mixed 
up by means of the stirrer, a thin paste is formed. This 
paste is run into a centrifugal machine, of which Fig. 22 
is an illustration. In the drum, B, provided with an out- 
flow, E, the smaller removable drum, C, with perforated 



WASHING APPAKATUS. 



123 



walls, is caused to rotate by gearing, G, to which it is 
attached by the screw, V. The drum, C, is lined by a tight 
linen bag. 

When the drum is in rapid motion, the thick liquid in 
the washing vessel is run in. In consequence of the centri- 
fugal force due to the rapid rotation, the whole mass is at 
once thrown on the sides of the drum, the liquid penetrates 
the fine openings, is caught in the outer vessel, and runs 




FIG. 22. 

away to a receiver, in which it is kept until the finest 
particles of the precipitate, which will penetrate even the 
closest fabric, have settled. The operation is continued 
until the excess of water has been separated, when the bag 
ontaining the precipitate is lifted out of the drum. Centri- 
fugalised enamel white is a fairly stiff, white paste, which 
should be packed in sacks lined with oiled paper to prevent 
drying. 



124 



MINEBAL AND LAKE PIGMENTS. 




WASHING APPARATUS. 125 

FILTER PRESSES. 

In recent years the use of filter presses for separating 
liquids from precipitates and for washing the latter has 
become general. A filter press consists, as shown in Fig. 23, 
of a number of frames, between which are perforated plates 
and sheets of filter cloth, and which can be pressed tight 
together by a screw. A powerful pump forces the liquid 
containing the precipitate into the hollow spaces of the 
frames (known as chambers), in which the solid body 
remains, whilst the liquid goes through the filter cloth. 
By afterwards pumping fresh water through, the solid 
remaining in the chambers is soon completely washed. 



CHAPTEE X. 

ZINC WHITE. 

ZINC OXIDE, known as a pigment under the name of zinc 
white, is one of the most important white pigments. Al- 
though not absolutely permanent, yet it has, in common 
with enamel white, the valuable property of preserving its 
whiteness in air containing sulphuretted hydrogen. Its low 
price has brought it into general use. 

Although zinc white is a most important pigment, it 
is very seldom made in colour works, because on account 
of its origin it is a product of metallurgical processes. Zinc 
white in chemical composition is pure zinc oxide ; it is 
formed when zinc vapour burns in air. In zinc-smelting 
works zinc white is obtained by putting zinc in tubes which 
are heated to whiteness ; the zinc vapours burn when they 
come in contact with the air, and the zinc oxide is caught 
by special arrangements. 

The retorts used for this purpose are similar to those 
employed in the manufacture of coal gas. From 8 to 18 of 
these retorts are arranged in a furnace, in two rows, one 
above the other. In the lid of the retorts is an opening, 
which serves for charging and for carrying off the zinc 
vapours. When the operation is commenced, the retorts 
being heated }to a white heat, two zinc plates are brought 
into each, the metal is soon volatilised, and the vapours pass 
through the above-mentioned openings. A current of air, 
heated to 300 C., is blown in to meet the zinc vapours, which 



ZINC WHITE. 



127 



take fire and burn with a blinding white flame, producing 
a very fine white powder, which is carried by the current 
of air through a series of chambers in which it deposits. 

C. Freitag recommends for the production of zinc white 
the use of retorts of oval section, A, Fig. 24. These retorts, 
containing crude zinc, are heated to a white heat, and then 
a mixture of generator gas from coke and air is introduced 
by B and the pipe running through B. The zinc burns com- 
pletely in the flame of the generator gas, which contains 




PIG. 24. 

excess of oxygen. A product of faultless nature is said to 
be obtained in this way. 

In zinc works zinc white is always made in the manner 
which has been described. It may also be obtained as a 
by-product in another metallurgical operation, the desilverisa- 
tion of lead by Parkes' process. In this process an alloy 
of silver and zinc is obtained. By sending a current of 
superheated steam over the molten alloy the zinc decom- 
poses the steam, hydrogen and zinc oxide being formed. 
The zinc white is carried by the current of gas into chambers,, 
in which it deposits. 



128 MINERAL AND LAKE PIGMENTS. 

The zinc white obtained by burning zinc is, as has been 
said, a very fine pure white powder, which can at once 
be used for paint without further preparation. The price 
of zinc white is rather higher than that of white lead, but 
the difference is counterbalanced by the greater covering 
power of the zinc white. Ten parts by weight of zinc white 
completely cover a surface for which 13 parts of white lead 
are required. 

Whilst white lead cannot be mixed with many pigments, 
such as those which contain sulphur, zinc white may be 
mixed with all without fear of alteration. Zinc white is 
even better than enamel white for producing pale pigments 
from lakes ; it has a lower specific gravity than enamel 
white, so that the mixture with the light lake can be 
more easily made. 

Zinc grey, which is produced by some works, is zinc 
oxide discoloured by metallic zinc. Pure zinc white always 
has a pure white colour ; if it is tinged with grey it is con- 
taminated by metallic zinc, whilst a brownish hue denotes 
the presence of cadmium oxide. The latter impurity will 
be rarely met with in commercial samples, since cadmium 
is worth much more than zinc. Zinc oxide is used by the 
colour maker in the preparation of Einmann's green ; it is 
also used, as stated above, as an addition to other pigments. 

Griffith's Zinc White. This pigment, which is equal in 
covering power to white lead, consists of zinc oxysulphide. 
It is obtained by precipitating a zinc solution with a solution 
of barium sulphide, washing, drying, igniting and grinding 
the precipitate. As it contains sulphur, it should not be 
mixed with copper or lead pigments. 

Tungsten White (Lead Tungstate), PbW0 4 , is obtained 
as a heavy powder by precipitating a solution of sodium 
tungstate with lead acetate and treating the precipitate, 
which consists of basic lead tungstate, with dilute acetic 



ZINC WHITE. 129 

acid, by which lead oxide is dissolved and a salt of the above 
composition left. This white pigment is dearer than other 
lead pigments, and has no special advantages over them ; 
it is, therefore, seldom used. 

WHITE ANTIMONY PIGMENTS. 

Antimony forms a number of white compounds which 
can be made by a simple and inexpensive process, and 
might, therefore, be used as pigments. Two antimony 
compounds in particular are so used antimony trioxide 
and oxy chloride (powder of algaroth). 

Antimony Trioxide occurs ready formed in nature a^ 
white antimony or antimony bloom. It is very simply 
prepared by burning the metal in air, when it forms soft 
needles with a silvery lustre. It is only necessary to heat 
melted antimony to a little above its melting point in a 
crucible placed in a slanting position, when the metal 
takes fire and burns with a blue flame. Nitric acid con- 
verts metallic antimony very quickly into antimony oxide 
with a copious evolution of brown fumes. 

Antimony trioxide may be more cheaply prepared from 
antimony sulphide, artificial or natural (stibnite), by finely 
powdering, moistening with water, and gently warming on 
plates. Oxidation takes place, the sulphur is converted into 
sulphur dioxide and the antimony to trioxide. The heating 
should not be carried too far, or the antimony takes up 
further oxygen and forms the tetroxide. 

Antimony Oxychloride, or Powder of Algaroth, is ob- 
tained by dissolving stibnite in strong hydrochloric acid, the 
operation being performed under a chimney to carry off the 
sulphuretted hydrogen. The solution of antimony trichloride, 
when the impurities have settled, is poured into a large vessel 
of water. At once a pure white precipitate is formed, which 
quickly sinks ; it is washed with water until the washings 



130 MINERAL AND LAKE PIGMENTS. 

have no acid reaction. Washing should not be continued 
too long or the oxy chloride will be further decomposed ; by 
washing with hot water it is almost entirely changed to 
antimony trioxide. The precipitate, after washing with cold 
water, has generally the composition expressed by the 
formula SbOCl. 

Antimony oxide and oxychloride are both very crystalline 
powders, and have in consequence small covering power. 

Bismuth White is not used as a painters' pigment ; it has 
no advantage over the white pigments previously described, 
and is much more expensive. Its only use as a pigment is 
for the preparation of white cosmetics, and even for this 
purpose zinc white is now frequently used ; it is cheaper 
and quite as satisfactory. 

Bismuth white is prepared by treating metallic bismuth 
with fuming nitric acid. The white precipitate at first 
formed completely dissolves in an excess of acid, and when 
the solution is poured into a large quantity of water, basic 
bismuth nitrate bismuth white separates. 

Pure bismuth white is a soft, heavy powder, brilliantly 
white ; it must be preserved in air-tight vessels as soon as it 
is dry, otherwise it acquires a yellowish tinge. Bismuth 
compounds are. if possible, more susceptible to the action of 
sulphuretted hydrogen than lead compounds. The yellow 
.colouration, turning to black in the course of time, is due 
to black bismuth sulphide. 

Tin White is used for earthenware enamels. It is 
obtained by treating granulated metallic tin with very 
strong fuming nitric acid. The heavy, white powder which 
is formed is separated from the undissolved tin by float- 
ing. Tin white has no application as a pigment ; when mixed 
with glazes it gives them a handsome, milky appearance. 

Manganese White. When large quantities of a solution 
of impure manganese chloride are at hand, such as are pro- 



ZINC WHITE. 131 

duced in the preparation of chlorine, manganese carbonate 
may be obtained. A small quantity of soda solution is first 
added and the liquid left for several days so that the oxide 
of iron may separate. When this is the case an addition of 
soda gives a pure white precipitate. 

Magnesia White or Mineral White is obtained, accord- 
ing to T. H. Cobley, by mixing a solution of magnesium 
sulphate with calcium chloride, adding 10 per cent, of 
aluminium chloride to the mixture and stirring in slaked 
lime so long as a precipitate is formed. A cheaper process 
is to precipitate mixed solutions of magnesium and alu- 
minium sulphates by slaked lime. 

Annaline. A white pigment is recommended for use 
under this name ; it consists of dead-burnt gypsum, which 
has been converted into a fine powder by grinding and levi- 
gating. (Dead-burnt gypsum has been so strongly heated 
that it is not able to again unite with water.) 

To obtain paler shades of certain colours, additions are 
made of natural pigments, such as chalk, which has been con- 
verted into a very fine powder by levigation. An addition of 
chalk to a heavy mineral pigment, such as chrome yellow, is 
not advisable ; it would, besides, seriously diminish its cover- 
ing power. 



In the preceding pages, a large number of white pig- 
ments has been enumerated. It would be easily possible to 
increase the list, but the result would be of no practical interest, 
for other pigments, neither in respect of quality nor price, 
can compete with the cheaper white pigments. Although 
white lead has at the present time an enormous use, it is to 
be hoped that this pigment, of good colour but little perman- 
ence, may be replaced entirely in the course of time by zinc 
white, and for some purposes by enamel white. 



132 MINEKAL AND LAKE PIGMENTS. 

White pigments, in addition to their use alone, are em- 
ployed to produce tints, which are obtained by mixing 
deeper colours with the white pigment. By adding the 
proper quantity of wiiite to a colour it is possible to produce 
all paler shades of that coloui . For example, the different 
varieties of the red lakes which are found in commerce are 
obtained by mixing white pigments with the deep red lakes. 

The particular white pigment to be employed in these 
mixtures depends on the nature of the colour and on its 
specific gravity. It should always be remembered that 
white lead will not increase the permanence of a colour, since 
it will be discoloured in a short time by the action of the 
atmosphere. In the manufacture of fine colours for artists 
lead pigments should be absolutely excluded. 

To produce paler shades of colours which contain lead 
and therefore have a high specific gravity, e.g., chrome yellow, 
white lead may be used ; for other colours, barytes or zinc 
white should be employed. In the case of lakes enamel 
white is too heavy ; zinc white or magnesia is suitable. 



CHAPTEK XI. 

YELLOW MINERAL PIGMENTS. 

As was the case with white pigments, so with yellow : of the 
large number known very few are in extensive use. In former 
times the number of yellow pigments employed in painting 
was far greater than at present ; several, formerly in general 
use, have dropped out, partially or entirely, owing either to 
their poisonous character or to their replacement by others, 
deeper and more handsome. Especially since the discovery 
of cadmium yellow and the development of the manufacture 
of chrome yellows, many colours once in general use have 
properly fallen into disuse. 

Again, unfortunately, the most important of the yellow 
mineral pigments contain lead, and have little stability ; but 
a series of yellow colours free from lead is known, and 
though some of them are inferior in shade to the lead pig- 
ments they surpass the latter in permanence. 

In addition to lead compounds, pigments derived from 
barium, zinc, antimony and cadmium are in general use. 
The yellow lead pigments were formerly preferred, and at 
present, so far as concerns beauty, they must be regarded 
as superior to other mineral yellows. The endeavour to 
provide the artist with permanent colours has resulted in the 
use of others, perhaps less brilliant, but very lasting. Under 
the name of chrome yellow a single pigment consisting of 
lead chromate was formerly understood ; to-day, under this 
title is comprised a series of pigments containing zinc or 
barium in place of lead, but all commercially known as 
chrome yellow. 



CHAPTEK XII. 

CHROME YELLOWS. 

THE chrome yellows are the lead, zinc or barium salts of 
chromic acid. In describing the different methods used in 
making these colours, lead chrome yellow will be taken first, 
since this is the one most largely used. 

LEAD CHEOME YELLOW. 

Both neutral and basic lead salts of chromic acid are 
known. Neutral lead chromate is found in nature as the 
somewhat rare mineral crocoisite, which is found in very 
small but perfectly shaped crystals in many lead mines. 

Neutral Lead Chromate, PbCrO 4 , is formed as a very 
heavy precipitate of a fine deep yellow colour when a solution 
of potassium chromate or bichromate is added to a solution 
of a lead salt in water. When exactly equivalent quantities 
of potassium chromate and lead solution are used, and the 
strength of the solutions is the same, the product has the 
same shade each time the operation is performed. It is not 
immaterial whether the one or the other salt is in excess, or 
whether strong or weak solutions are used ; all these con- 
ditions modify the shade of the chrome yellow produced. 
Many colour makers are apparently of the opinion that 
some particular skill of the workman is necessary to produce 
chrome yellow of a particular shade. This is, however, not 
the case; manufacturers who know the simple conditions 
which are important in making chrome yellows may produce 
any desired shade without difficulty. 



CHKOME YELLOWS. 135 

Neutral lead chromate readily parts with half the chromic 
acid it contains. When treated with alkalis, such as lime or 
caustic soda, or even when digested with finely ground lith- 
arge, the neutral salt gives up half of its chromic acid, and is 
converted- into the basic chromate or chrome red, Pb^CrO-. 

Basic lead chromate has, as the name chrome red indi- 
cates, a fine red colour. If the quantity of lime or caustic 
soda used is sufficient to decompose only a portion of the 
neutral lead chromate, a mixture of the yellow neutral 
and the red basic chromate is formed, the shade of which 
will incline to yellow or red according as it contains a pre- 
ponderance of one or the other compound. The pigment 
known as orange chrome is a mixture of approximately equal 
parts of the neutral and basic lead chromates. 

In order to brighten the deep yellow shade which dis- 
tinguishes neutral lead chromate, it is either mixed with a 
white pigment, or a white substance (lead sulphate) is pre- 
cipitated from the solution simultaneously with the lead 
chromate. In this manner all the imaginable pale yellow 
shades, lemon yellow, sulphur yellow, etc., can be obtained. 

Just as the quantities of the solutions used and their 
strength influence the shade of the chrome yellow they 
produce, so it also appears to be not immaterial which lead 
salt is employed. Colour makers are generally agreed that 
the finest product is obtained from neutral lead acetate. 
Any lead salt, even insoluble in water, may be used for 
the preparation of chrome yellow. The affinity of chromic 
acid for lead is so great that an interchange of constituents 
occurs between the insoluble salt and the potassium chromate. 
Chrome yellow may be made from lead acetate, chloride or 
sulphate ; the resulting substances are the same, but there 
is a considerable difference in regard to the beauty of the 
product. The finest chrome yellow, which leaves nothing 
to be desired in beauty of shade, is obtained by proceeding 



136 MINEEAL AND LAKE PIGMENTS. 

in the following manner : Lead acetate is dissolved in water, 
the solution diluted with an equal volume of water, and then 
mixed, under constant stirring, with a similarly diluted solu- 
tion of potassium chromate or bichromate. The precipi- 
tate is immediately formed, and quickly sinks to the bottom 
in consequence of its high specific gravity. It is washed 
with clean water so long as this removes soluble salts. The 
precipitate is then drained on cloths and dried in the air. 

The finest product is obtained by working with the fol- 
lowing proportions : 

Sugar of lead 100 

Potassium bichromate, or .... 50 

Potassium chromate . . . . . 40 

If lead sulphate is used the following quantities are to be 
employed : 

Lead sulphate . . . . . . 100 

Potassium bichromate ..... 25 

In the case of lead chloride the following is the propor- 
tion : 

Lead chloride . . .... 100 

Potassium bichromate ..... 27 

The chrome yellows prepared from insoluble lead salts have 
no particular beauty, but they may be used for mixed colours 
such as the spurious chrome green. 

Preparation of the Lead Solution, Many makers of 
chrome yellow do not use commercial lead acetate, but 
prepare its solution themselves. The preparation of this 
solution requires neither much space nor labour, so that 
considering the high price of lead acetate this procedure 
may be regarded as advisable, but only when acetic acid is 
obtainable at a low rate. 

The following is the method by which lead acetate solu- 
tion is made. Lead is granulated by pouring the molten 
metal from a height of several yards' into cold water, which 



CHEOME YELLOWS. 137 

is kept in rapid motion. The smaller the particles of lead, 
the larger surface they will possess, and the more quickly 
they will dissolve. For the solution of the lead small tubs 
are used, 50 centimetres in diameter and 90 to 100 centi- 
metres in height, provided with a tap immediately above 
the bottom to run off the liquid. Four of these vessels are 
so placed, one above the other, that the contents of each 
may be run into the one next below it. The lead in the top 
vessel is covered with acetic acid ; in a few minutes this is 
allowed to flow into the second vessel, and similarly after 
a, few minutes from the second to the third, from the third 
to the fourth, from which it runs away into a receiver below. 
After this treatment it contains but a small quantity of lead 
acetate ; the object of the operation is to start the oxidation 
of the lead, which quickly follows when air has sufficient 
access, the lead particles lose their metallic appearance 
and become covered with a white layer. When this is the 
case, the acetic acid is pumped from the receiver back into 
the top vessel, where it is left one to two hours in contact 
with the lead. It is then run off into the second, and thence 
into the third and fourth, remaining in each vessel for about 
the same time ; the resulting liquid is an almost completely 
neutral solution of lead acetate. The treatment with acetic 
acid is continued so long as lead remains undissolved. 

The solution of potassium chromate is prepared in a tub. 
The salt is easily soluble, and warming is unnecessary if 
the chromate is placed in a basket, lined with close linen 
cloth, hung in the liquid so that it is immersed to half its 
depth. The salt rapidly dissolves, its solution has a greater 
density than water, in consequence of which it sinks and 
fresh quantities of water continually come in contact with 
the salt. 

Precipitation of the Chrome Yellow, Before the chrome 
yellow can be precipitated it is necessary to estimate the 



138 MINERAL AND LAKE PIGMENTS. 

quantity of lead acetate contained in the solution, since upon 
this depends the quantity of potassium chromate solution to 
be used. If the lead solution contained only acetate and 
water, its strength could be simply found by means of the 
hydrometer. It contains, however, varying quantities of 
acetic acid, on which account the hydrometer would give 
very inaccurate results. The test by which the relation 
between lead solution and potassium chromate solution is 
found is performed in the following manner : The lead 
solution is measured off in a cylinder divided into 100 divi- 
sions ; the same volume of potassium chromate solution 
is measured and placed in a high narrow vessel ; the lead 
solution is gradually added to the chromate solution so long 
as a precipitate is formed. The precipitate settles rapidly, 
and there is no particular difficulty with a little practice 
in finding with sufficient accuracy the quantities required 
for the precipitation. In order to precipitate 100 litres of 
potassium chromate solution, there are required as many 
litres of lead solution as were used divisions of the cylinder. 

The preparation of the chrome yellow is now a very 
simple matter. Whilst steadily stirring, the measured 
quantity of lead solution is run into the solution of potas- 
sium chromate ; the precipitate is allowed to settle, is well 
washed and dried. It does not make any difference to the 
colour whether potassium chromate or bichromate is used ; 
the same product is obtained in each case. 

It is stated by Dullo that chrome yellow prepared by the 
preceding process alters in colour on long keeping. This is 
ascribed to the formation of a basic compound. According 
to the same author, a chrome yellow free from this objec- 
tionable property is obtained by using lead nitrate in place of 
acetate and an excess of potassium chromate solution. The 
writer has kept chrome yellow, made from lead acetate, for 
years without observing the slightest alteration in the colour. 



CHROME YELLOWS. 139 

On chemical grounds, it is incomprehensible that a chrome 
yellow prepared from lead nitrate should have different 
properties to the same substance prepared from another 
soluble lead salt, and freed from foreign substances by suffi- 
cient washing. 

The product of this process is that which par excellence 
is known as chrome yellow, the chemist's neutral lead 
chromate ; it exhibits a characteristic deep yellow colour, 
a shade which is known as chrome yellow. Under the 
microscope chrome yellow is seen to be a crystalline mass ; 
it will possess greater covering power the smaller the 
crystals. Now, the motion of a liquid in which crystals 
are forming prevents the production of large crystals, thus 
the reason is clear for the rapid stirring of the solutions 
in the preparation of this pigment. 

According to C. 0. Weber, who has published an ex- 
haustive account of chrome pigments (Dingier 8 Journal, 
282), the cost of the lead chrome pigments varies greatly 
according to the raw materials employed. Assuming that 
100 kilogrammes of litharge cost 35 marks, 100 kilogrammes 
of 30 per cent, acetic acid 25 marks, and 100 kilogrammes 
of 60 per cent, nitric acid 26 marks, Weber calculates that 
100 kilogrammes of litharge, in a form suitable for making 
chrome yellow, will cost as follows : 

From lead acetate at 56 marks per 100 kilogrammes . . 96 marks 
,, ,, solution made in the works . . .80 

,, lead nitrate at 50 marks per 100 kilogrammes . . 75 
solution made in the works . . .64 

By the basic lead acetate method 51 

,, ,, chloride ,, . . . . . .40 

White lead method 55 

The Pale Chrome Yellows. When the solution of the 
chromate used for the precipitation of the lead solution is 
mixed with sulphuric acid, then a mixture of lead sul- 
phate and lead chromate is formed on precipitation. Lead 



140 MINEEAL AND LAKE PIGMENTS. 

sulphate is white, so that the colour of the precipitate would 
be paler according to the quantity of sulphuric acid added to 
the potassium chromate solution. There are, however, com- 
pounds of lead chromate and lead sulphate, of which we know 
two. Their composition is expressed by the formulae : 

PbCrO 4 . PbSO 4 and PbCrO 4 . 2PbS0 4 . 

The former is a beautiful lemon yellow shade, the latter 
nearly approaches sulphur yellow. By corresponding alter- 
ations in the quantity of sulphuric acid added, all inter- 
mediate shades can be obtained. On the works these shades 
are made in the following manner : buckets are used for 
taking the potassium chromate solution out of the vessel in 
which it was made ; these buckets hold 12'5 litres. Now, if 
the solution of potassium chromate has been made from 
25 kilogrammes of the salt and 750 kilogrammes of water, 
one of these buckets holds exactly 0'43 kilogramme of 
chromic acid. In order to obtain the lemon yellow com- 
pound, 0'39 kilogramme of sulphuric acid must be added 
to a bucketful of solution ; 0*78 kilogramme must be added 
to obtain the sulphur yellow chrome. These liquids are pre- 
pared by pouring the sulphuric acid in a thin stream into 
the potassium chromate solution made as above, the liquid 
must be stirred whilst the sulphuric acid is being added. 
The lemon chrome has the peculiar property of increasing 
considerably in volume soon after formation, a property to 
which regard must be had in the manufacture. A descrip- 
tion of the rational preparation of the lemon and sulphur 
yellow shades of chrome yellow on the manufacturing scale 
follows. 

In making lemon chrome, the tub in which the precipi- 
tation is to take place is two thirds filled with water, the lead 
solution is stirred in, and then the chromate solution, mixed 
with the proper quantity of sulphuric acid, is run in ; the 



CHEOME YELLOWS. 141 

liquid is well stirred so that the precipitate may form as 
quickly as possible throughout the whole liquid. The pre- 
cipitate is allowed to settle, the liquid drawn off, and the 
colour washed twice with water as quickly as possible. The 
paste is then removed from the tub and poured on a strong 
linen strainer. At first, the fine precipitate goes through the 
strainer, the liquid is poured back on the strainer until the 
size of its pores is so far diminished by the precipitate itself 
that only clear liquid runs through. The precipitate is left 
on the strainer until it forms a stiff mass, which can be 
easily spread out upon boards by spatulas. 

To obtain a good, that is, a loose product, it is necessary 
to carry out the processes so quickly that the swelling men- 
tioned above does not. take place while the colour is being 
strained, but when it has been spread out on the boards. 
This swelling only takes place completely when the layer of 
precipitate is fairly thin. Large boards should be used, upon 
which the precipitate is spread out in a very thin layer. To 
prevent the mass which is still fairly fluid from running 
off the boards, they are provided with raised edges, and the 
paste is spread out smoothly in these flat trays. If the 
operations have been properly performed, the precipitate at 
once begins to swell and becomes of a loose nature. When 
it has acquired a buttery consistency it is cut up, by means 
of a thin sheet of brass, into prisms, which are placed near 
one another standing on the narrow side, and dried first in 
the shade and then in the sun. It is necessary that the dry- 
ing should take place slowly at first, or the cakes will crack 
or even fall to pieces. The precipitate cannot be washed 
completely in the tub, because it often begins to swell on 
the strainers, consequently a crystalline crust covers the 
surface of the cakes during drying. This layer must be 
removed by scraping the cakes of colour, in which operation 
small quantities of chrome yellow dust become suspended in 



142 MINERAL AND LAKE PIGMENTS. 

the air. To protect the workman against poisoning by this 
lead compound, precautions must be taken against breathing 
in the dust. The simplest and most efficacious is to tie a wet 
sponge over mouth and nose ; this retains the particles of 
dust in the inspired air. The dust, scraped off the cakes, 
is put into water, in which the salts dissolve, whilst the 
chrome yellow sinks to the bottom. In this process for 
preparing chrome yellow the solution of potassium acetate 
left after the precipitation contains a considerable quantity 
of free acetic acid, which may be utilised to dissolve lead. 

When sulphur yellow chrome is to be made, the process 
is substantially the same as for the preparation of the lemon 
shade, but with the difference that everything is done to 
prevent the swelling of the precipitate. The precipitation 
and washing of the precipitate are done as quickly as possible ; 
the washed precipitate is filled into press bags and strongly 
pressed. Care must be taken in the pressing that the press- 
ure is only gradually increased ; if powerful pressure is 
applied at once, even the strongest cloths will be burst. 
The more thoroughly the precipitate is pressed, the closer 
will be the fracture of the chrome, a property which is re- 
garded as a sign of good quality in this species of chrome 
yellow. 

The manufacture of chrome yellow is intimately con- 
nected with that of a number of colours, varying from orange 
to dark red, which are known under the names of chrome 
orange or chrome red. In accordance with the division 
adopted of pigments according to their colour, chrome orange 
and chrome red will be considered among the red mineral 
pigments. 



CHAPTEK XIII. 

LEAD OXIDE PIGMENTS. 

LEAD MONOXIDE, PbO, exists in two different modifications ; 
in the crystalline form, as litharge, which is generally pale 
yellow with a reddish tinge, and amorphous, as massicot, 
which is yellowish red. 

Lead monoxide is a product of metallurgical works rather 
than of colour works ; still the preparation of the crude oxide 
falls in the domain of the colour maker, since from litharge 
several pigments may be prepared by a simple treatment. 

Massicot is obtained by heating white lead, lead nitrate 
or red lead, and also by heating melted lead in the air, with 
the precaution that the oxide formed does not itself melt. 

Litharge is obtained as a by-product in several metallur- 
gical processes, such as the cupellation of lead containing 
silver, in which the lead is melted in a furnace with a shallow 
hearth and a powerful current of air blown over the melted 
metal. The lead is oxidised, the oxide melts at the high 
temperature approaching 1000 C., and flows through an 
orifice in the wall of the furnace, whilst the silver remains on 
the cupel. The litharge is ground and levigated and, accord- 
ing as it is pale yellow or reddish, sold under the name of 
silver or gold litharge. 

Both massicot and litharge have no particularly striking 
colour ; they are seldom used as pigments. Litharge has 
an extensive use in oil boiling and for the manufacture of 
lead peroxide, which is used for matches. 



144 



MINERAL AND LAKE PIGMENTS. 



Red Lead, Minium. Lead forms a number of other 
oxygen compounds, one of which, red lead, has the composi- 
tion, Pb 3 O 4 . It is a bright red powder, used as a pigment 
and as a constituent of certain cements (for gas and water 
pipes). 

Bed lead is, like litharge and massicot, a metallurgical 
product, but, by working on a small scale, products can be 
obtained of a much brighter colour than the produce of the 
large scale. 

Bed lead is made in two ways directly from metallic 




FIG. 25. 

lead or by heating easily decomposed lead salts. When it 
is made from the metal, the following is the process : the 
lead, which must be very pure, is melted in a reverberatory 
or calcining furnace, oxidised to massicot by the air passing 
over it, and the massicot then, by careful heating, changed 
into red lead, a process in which particular care must be taken 
that the mass is not melted. By continued heating the 
massicot absorbs about 2 per cent, of oxygen, and changes 
in colour to a bright red. 

The art in making red lead by this process lies in main- 



LEAD OXIDE PIGMENTS. 



145 



taining the proper temperature ; the furnaces are constructed 
so that the temperature may be regulated during the heating. 
A reverberatory furnace is used, in which is a stirring appara- 
tus, so that the heated mass may be continually turned over 
to accelerate the oxidation. Muffle furnaces are also used, 
in which the massicot is placed in crucibles on an iron plate, 
which can be pulled out of the furnace for observation of 
the change of colour. Whatever method is used, the tem- 
perature must be so regulated that overheating of the material 
is avoided, otherwise litharge is formed, which is only very 
slowly converted into red lead. 

Mercier states that the muffle furnaces are arranged as 




FIG. 26. 

shown in Fig. 25. The muffle, a, is 2*5 metres long and 2 
metres wide ; its bottom rests on an iron plate. The pas- 
sage, d, running under the muffle is 20 centimetres high ; it 
is divided by a partition, and at each end are two hearths,, 
c, 70 centimetres long and wide. The products of combustion 
pass from the long passages into side channels,/, provided with 
dampers, go round the muffle and unite in the space, g. The 
flue, k, at the back of the muffle is provided with dampers, m, 
which exactly regulate the current of air through the muffle ; 
n is the chamber in which are collected the particles of oxide 
carried over by the draught. The furnaces used for manual 
labour consist, according to Percy (Figs. 26 and 27) of a 

rectangular or circular hearth, a, of about 3 metres diameter, 

10 



146 



MINERAL AND LAKE PIGMENTS. 



which is deeper in the middle and has two fireplaces, b. The 
low arch surmounting the hearth is covered with sand in 
order to prevent cooling. 

When finished, the red lead is drawn out of the furnace 
and finely ground under edge-runners, or occasionally levi- 
gated. The temperature necessary in making red lead ia 




FIG. 27. 

that at which the angles of the muffles, when these are used, 
begin to show a dark red glow. 

Orange lead, which is a brighter variety of red lead, is 
prepared from lead salts ; white lead or lead nitrite is used 
for this purpose. The latter salt is made by the process of 
Pischon, by heating 1 equivalent of lead nitrate with 4 
equivalents of granulated lead and water at a temperature 



LEAD OXIDE PIGMENTS. 147 

between 50 and 60 C. After about 2 hours, the lead 
nitrite separates in the form of a granular yellow mass. 
According to Burton's process, lead carbonate is oxidised by 
heating with 20 per cent, of sodium nitrate and extracting 
the mass with water. There are also other methods by which 
red lead is obtained from litharge by the use of potassium 
chlorate or saltpetre ; but these methods, without producing 
a finer product than those previously given, are more ex- 
pensive, and consequently have found no application on the 
large scale. 



CHAPTEE XIV. 

OTHER YELLOW PIGMENTS. 

Cassel Yellow, also known as mineral or Veronese yellow, 
has now a very restricted use ; it has been replaced by the 
deeper and cheaper chrome yellow. Much of the Cassel 
yellow of commerce is nothing but chrome yellow shaded 
with barytes. As regards chemical composition, Cassel 
yellow has the following formula: PbCl 2 .7PbO. It is ob- 
tained by heating litharge, red lead or white lead with 
ammonium chloride. To 10 parts of the lead compound 
1 part of ammonium chloride is used ; on melting, am- 
monia is set free, by which part of the lead oxide is decom- 
posed, metallic lead separating. The melted mass is poured 
off from the lead into iron moulds, in which it solidifies to 
a very crystalline substance of a fine yellow colour. By 
grinding and levigating, the Cassel yellow is prepared for use. 
Pale yellow shades, obtained by admixtures of barytes, are 
occasionally encountered. 

Montpellier Yellow consists, like the preceding pigment, 
of basic lead chloride. It is obtained by gradually mixing 
400 parts of powdered litharge with a solution of 100 parts 
of common salt in 400 parts of water. After each addition 
of salt solution the pasty mass must be thoroughly stirred, 
or it will harden. When all the salt solution has been 
mixed with the litharge to a homogeneous white mass, the 
latter is treated with water to remove excess of salt, and the 
washed material dried and melted in earthenware crucibles. 



OTHEE YELLOW PIGMENTS. 149 

The melt, which has a bright yellow colour, is groiind and 
levigated, when it forms a handsome pigment. 

There are several other yellow pigments of similar com- 
position, of which one only need be mentioned, obtained by 
treating a solution of zinc chloride with lead hydroxide. 

Turner's Yellow or English Yellow is prepared by two 
methods : either by melting 7 parts of finely ground 
litharge with 1 part of common salt ; or by treating lith- 
arge with a solution of common salt and converting the 
white oxychloride into a yellow pigment by melting. 

Naples Yellow, This handsome pigment, which is, unfor- 
tunately, susceptible to the action of sulphuretted hydrogen, 
is known commercially under different names. Naples yellow 
takes its names from the fact that it was formerly exclusively 
made in Italy, where the method was kept secret, a secret 
which disappeared with the advance of analytical chemistry. 
Naples yellow is now known to be lead antimoniate. 

Naples yellow is a handsome pigment. Its preparation 
is more tedious than that of chrome yellow, hence it is now 
rarely employed. The author has had practical experience 
that much of the so-called Naples yellow of commerce is 
nothing but a suitably shaded chrome yellow. 

Naples yellow can be prepared by different methods. 
According to the oldest, given by Brunner, 1 part of pure 
tartar emetic is carefully and thoroughly ground with 2 
parts of lead nitrate and 4 parts of common salt. The 
mixture is melted at a low heat in a Hessian crucible, and 
the fluid mass poured on a cold iron plate. After cooling, it 
is boiled out with water, when lead antimoniate remains as 
a powder of a more or less deep yellow colour. It is not 
easy to obtain this favourable result with certainty in every 
case. If a certain temperature is exceeded only by a little, 
a hard mass results, which by long boiling does not become 
a fine pow r der, but a sandy substance of little brilliance. Even 



150 MINEBAL AND LAKE PIGMENTS. 

when the operation succeeds, the product often varies con- 
siderably in shade, sometimes a sulphur yellow, at other 
times an orange pigment being formed. As a rule, the paler 
product is obtained at a lower temperature ; by stronger 
heating, darker products of a red shade are obtained. 

According to another recipe, 2 parts of tartar emetic 
are melted with 4 parts of lead nitrate and 8 parts 
of common salt. The mass is treated with very dilute 
hydrochloric acid for a long time, which extracts some 
quantity of lead oxide, a deeper product being thus obtained. 
Care is, however, necessary in this treatment ; acid of too 
great strength would spoil the whole product. 

The Paris method for Naples yellow is as follows : metallic 
antimony is oxidised by melting in air ; to 12 parts of 
antimony, 8 parts of red lead and 4 parts of zinc oxide 
are used, and the mixture is melted at a low red heat. 

A cheap, but not particularly bright product, can be ob- 
tained from old printer's type. The metal, which is an alloy 
of antimony and lead, is powdered, mixed with 3 parts 
of saltpetre and 4 parts of common salt, melted, and the 
mass washed out with water. 

Other formulae which are said to yield a good result 
are as follows : 12 parts of white lead, 3 parts of anti- 
mony oxide, 1 part of ammonium chloride, 1 part of 
alum. Or : 16 parts of stibnite, 24 parts of lead, 1 part 
of common salt and 1 part of ammonium chloride. The 
intimate mixture of these materials is first gently heated 
with access of air, then more strongly, and the mass 
extracted with water. There are many other recipes for 
the preparation of Naples yellow, the majority of which 
are distinguished by an apparently arbitrary arrangement 
of the materials ; for there is no scientific reason. If it 
were possible to accurately obtain any desired high tem- 
perature in a furnace, the manufacture of Naples yellow 



. OTHBE YELLOW PIGMENTS. 151 

would no longer be a matter of skill, but the same product 
could be obtained at every attempt. Since this is not yet 
the case, the exact procedure for the preparation of this 
colour can only be found by careful experiments. 

Naples yellow is, as has been said, a handsome colour, 
and offers a great resistance to varied reagents. It is only 
changed by one of them, sulphuretted hydrogen, by the 
prolonged action of which it is turned completely black. 

Antimony Yellow is very similar in composition to Naples 
yellow. It consists of a mixture of lead antimoniate with 
the oxides of lead and bismuth. It is prepared by the pro- 
cess recommended by Merome by intimately mixing 3 parts 
of finely powdered bismuth with 24 parts of powdered 
stibnite and 64 parts of saltpetre, melting the mixture and 
shaking it whilst molten into water. The brittle mass is 
finely powdered, washed and dried, then melted with 128 
parts of litharge and 8 parts of sal ammoniac. The mass 
obtained has a fine pale yellow colour ; when powdered it is 
antimony yellow. This pigment has almost fallen into 
disuse because of its instability and the high price of 
bismuth. 

Calcium Chrome Yellow. Calcium forms a yellow pig- 
ment with chromic acid, which, although far surpassed by 
the lead chromes in fineness of shade, has the advantage over 
them of greater stability and cheapness. For purposes for 
which cheap and at the same time permanent colours are 
required calcium chrome yellow can be recommended. It 
is most simply prepared from potassium chromate and cal- 
cium chloride, which, as a by-product of many chemical 
operations, is obtainable at very low prices. The deepest 
pigment is obtained when the precipitation is done with a 
boiling solution of the chromate. Calcium chromate, in 
addition to its use alone, may be employed instead of white 
pigments to produce pale shades from deep lead chromes. 



152 MINERAL AND LAKE PIGMENTS. 

This addition should not be carried to a great extent or the 
chrome will be made too light, since calcium chromate has 
a much lower specific gravity than lead chromate. 

Barium Yellow, Yellow Ultramarine or Permanent Yellow. 
This pigment consists of barium chromate. The finest 
product is obtained when a solution of a barium salt, generally 
barium chloride, is precipitated boiling by a solution of 
potassium chromate. The very finely divided precipitate has 
a pale yellow colour very similar to that of pale lead] chromes. 
This handsome pigment is distinguished by the valuable 
property of being practically unaltered by the atmosphere ; 
it is only attacked by strong acids and alkalis. By long 
heating, the colour of this compound is gradually changed to 
a handsome green, which consists of a compound of barium 
and chromic oxides, and has occasional use as an artists' 
colour. In order to obtain this pigment, the heating must 
be intense and long continued. According to the author's 
experiments, it is not sufficient to heat for a short time to a 
very high temperature ; in that way a mass is obtained of 
very unequal colour. The best result was obtained by 
spreading barium yellow in a thin, even layer in a flat 
porcelain dish and heating to whiteness for 10 hours. 

Zinc Chrome Yellow, Zinc chromate is inferior to lead 
chromate in beauty, but has the advantage of permanence. 
It does not blacken in an atmosphere of pure sulphuretted 
hydrogen, and resists very well the action of other agents. 
Zinc yellow may be prepared by the immediate precipita- 
tion of a solution of zinc sulphate by a solution of potassium 
chromate, both being boiling, but the very bright precipitate 
obtained in this way is not stable; on washing, it gives up 
chromic acid continually to the wash water, and only a 
pale yellow residue remains. A very fine colour is ob- 
tained in the following manner : zinc sulphate is dissolved 
in water and boiled for half an hour with 1 per cent, of 



OTHER YELLOW PIGMENTS. 153 

white zinc whilst stirring. This operation effects the 
separation of iron oxide and the neutralisation of the 
free acid generally present in commercial zinc sulphate. 
When the solution has cleared by standing, it is precipitated 
by a solution of potassium chromate, the precipitate col- 
lected on a filter and allowed to drain completely; it is 
then washed with very small quantities of water and finally 
dried. A pure yellow precipitate is only obtained when all 
the iron oxide has been removed by boiling the zinc sul- 
phate solution with white zinc ; if the liquid contains only a 
very small quantity of iron, it has yet a very considerable 
influence on the colour, the yellow is not pure, but has 
a brownish tinge. Zinc yellow is used alone, and mixed 
with other pigments. Chrome yellows of all possible shades 
may be obtained in this way. Chrome yellows are often 
found in commerce which consist essentially of zinc chromate. 

Cadmium Chrome Yellow, When a solution of cadmium 
sulphate, or any other cadmium salt, is mixed with a solution 
of potassium chromate, a precipitate of cadmium chromate, 
CdCrO 4 , is formed. This pigment has a beautiful, deep 
yellow colour, in no way inferior in shade to the finest lead 
chromes, and having the great advantage over the latter of 
being entirely unaltered by the atmosphere ; it is thus to be 
highly recommended for artistic purposes. The high price 
at which it is sold prevents its general use, though now that 
cadmium compounds are to be obtained at so much lower 
prices than formerly, the price of cadmium chromate appears 
to be excessively high. 

Cadmium Yellow is cadmium sulphide, CdS ; in nature 
it occurs as the somewhat rare mineral greenockite. Cad- 
mium yellow is obtained by dissolving metallic cadmium 
in sulphuric acid, and precipitating the solution with sul- 
phuretted hydrogen. The solution of cadmium sulphate 
must be digested for some time with excess of cadmium, in 



154 MINEKAL AND LAKE PIGMENTS. 

order to separate the foreign metals present as impurities ; 
the colour is not so fine when a quite pure cadmium solution 
is not used. 

Cadmium yellow is a very bright yellow. Several shades 
are obtained according as the solution of cadmium sulphate 
used in its precipitation is neutral or acid. The reason of 
this difference in shade lies apparently in the different size 
of the crystals of which the precipitate is composed. The 
deep, pure yellow colour becomes still deeper by fusion, 
which takes place at a white heat. Weak alkalis, acids and 
sulphuretted hydrogen do not alter cadmium yellow ; it is 
thus to be regarded as a durable artists' colour. It can be 
mixed with ultramarine without decomposition, when a fine 
green is formed ; but mixed colours cannot be made from 
cadmium yellow and blue copper pigments, since these would 
blacken in the light. 

Lead Iodide. On precipitating a solution of lead nitrate 
with potassium iodide, lead iodide is formed. This is but 
slightly soluble in water, and, when dry, has a handsome, 
deep yellow colour. Unfortunately it is not permanent, but 
is decomposed on exposure to light. It can be used for 
bronzing, but other and cheaper pigments are available for 
this purpose. 

On account of the great solubility of lead iodide in a 
solution of potassium iodide, it is prepared in another way, 
and accurately weighed quantities are used. Calcium iodide 
may be used instead of the potassium salt ; 100 parts of 
iodine, 15 parts of fine iron filings and 25 parts of lime are 
mixed with sufficient water to form a thin paste, which is 
warmed until all the iodine is dissolved, when water is added, 
the liquid filtered, and the residue washed in order to extract 
all the calcium iodide. The solution and wash waters are 
united, then a solution of 152 parts of lead acetate is added, 
when all the iodine is precipitated as lead iodide. 



OTHEK YELLOW PIGMENTS. 155' 

A simpler method is to dissolve equal parts of lead nitrate 
and potassium iodide separately, each in 20 parts of hot 
water, to mix the solutions and cool quickly, when lead iodide 
separates in very small crystals. When pure lead iodide is 
melted in the absence of air, and the fused mass powdered,, 
a product of yet finer colour is formed. It is necessary to 
completely imbed the crucible in which the fusion is per- 
formed in the fire. The action of air on the melted mass- 
would produce a basic iodide. The fine golden yellow colour 
of lead iodide adapts it especially for the production of gold 
bronzes on wall papers and fabrics. 

Mars Yellow, which is generally reckoned among the- 
best artists' colours, is usually a mixture of ferric oxide and 
calcium sulphate or alumina. The pigment is prepared by 
mixing a solution of ferrous sulphate with milk of lime, 
when ferrous oxide is precipitated, which becomes yellowish 
brown on exposure to air, in consequence of the oxidation 
of the ferrous oxide. By heating the precipitate, according 
to the temperature different shades are obtained, varying 
between yellow and red. In addition to Mars yellow, Mars 
orange and Mars red are found in commerce. 

The manufacture of this pigment is very simple : 1 part 
of ferrous sulphate is dissolved in 10 parts of water, and 
the solution mixed with milk of lime made from 1 part of 
quicklime and 40 parts of water. If it is desired to produce 
a darker shade, and especially a product to be afterwards 
converted into Mars orange, the amount of ferrous sulphate 
is increased to 2 parts. When the mixture has been made, 
it must be stirred for a long time, in order that the reacting 
substances may come thoroughly into contact. The pre- 
cipitate, which at first is greenish grey, soon acquires by 
the action of the air the colour of ferric hydroxide, which 
becomes deeper on drying. 

When dried and finely ground Mars yellow is heated in 



156 MINERAL AND LAKE PIGMENTS. 

thin layers, it changes to dark yellow, and finally to orange 
red, a similar alteration taking place to that occurring when 
ferric hydroxide itself is heated. 

A Mars yellow of a deeper shade, consisting of a mixture 
of ferric hydroxide and alumina, is obtained by precipitating 
with caustic soda a solution of ferrous sulphate and alum. 
The sodium sulphate which is formed at the same time 
must be removed as completely as possible by washing with 
boiling water. 

By calcining Mars yellow for a long time at a high 
temperature, Mars brown is produced, a fine brown pig- 
ment. The value of Mars yellow and the pigments obtained 
from it lies not only in their fine shade, but in their perma- 
nence, which distinguishes the majority of the iron colours. 

Siderin Yellow. This not very handsome yellow consists 
of ferric chromate ; it is obtained by adding a neutral solu- 
tion of ferric chloride to a strong boiling solution of potassium 
bichromate so long as a precipitate is formed. Siderin yellow- 
is said to be used both in oil and water, and to be particu- 
larly adapted for use in sodium silicate paints, since in the 
course of time it forms a stony mass with that substance. 

The low price of iron salts would make it desirable to 
employ chromates of iron, but it appears to be difficult to 
obtain a compound of constant composition. In experi- 
ments with this object the author did not succeed in 
obtaining products of the same shade. Others have pro- 
bably been equally unsuccessful, for siderin yellow has 
never been used in quantity, as it would have been were 
there no difficulties in the way of its preparation. 

Aureolin is a double nitrite of cobalt and potassium, 
Co(N0 2 ) 2 .3 KN0 2 . This pigment is prepared by adding 
excess of potassium nitrite to a solution of cobalt nitrate 
acidified with acetic acid. As the liquid cools, a deep lemon 
yellow crystalline powder separates, which, when dry, is 



OTHEB YELLOW PIGMENTS. 157 

known as Indian yellow or aureolin. It is distinguished from 
other yellow pigments by being unaffected by sulphuretted 
hydrogen. 

The potassium nitrite required in the preparation of this 
pigment is most easily made by melting saltpetre in a thick 
iron vessel and stirring in fine iron filings in small quantities 
as soon as the saltpetre begins to decompose. The iron 
glows brightly and burns to oxide, the saltpetre changes 
to potassium nitrite. The mass is dissolved in a little hot 
water, the solution filtered and cooled, when most of the 
undecomposed saltpetre crystallises out, whilst the nitrite 
remains in solution. After further evaporation and separa- 
tion of another crop of potassium nitrate crystals, the 
solution can be used to precipitate the aureolin. 

It is advisable to use strong solutions in the precipitation 
of aureolin ; the finest precipitate is obtained in this way. 
If dilute solutions are used, the precipitate forms gradually ; 
it is then coarse and has little covering power. 

According to the method of Hayes, aureolin is prepared 
by passing into a solution of cobalt nitrate the vapours pro- 
duced by pouring nitric acid over copper and allowing air 
to enter. Caustic potash is added to the liquid from time 
to time. In this way all the cobalt can be obtained in the 
form of a yellow precipitate. 

Tungsten Yellow. Finely powdered tungsten is intro- 
duced in small quantities into fused potassium carbonate so 
long as effervescence occurs. After boiling with water and 
filtering, calcium tungstate is precipitated from the filtrate 
by means of calcium chloride. The moist precipitate is added 
to hot dilute nitric acid until the liquid is only slightly acid, 
when it is boiled for half an hour and allowed to cool. The 
precipitate, after washing with a little water and drying, is a 
deep lemon yellow powder. 

Nickel Yellow consists of nickel phosphate. It is obtained 



158 MINERAL AND LAKE PIGMENTS. 

by adding sodium phosphate to a solution of nickel sulphate 
or nitrate, and heating the pale green precipitate to redness. 
Nickel yellow has a pleasing shade, and is distinguished by 
great permanence. Up to the present it has found little 
use as an artists' colour, but on account of its permanence, 
which does not distinguish many yellow pigments, its use 
is to be recommended. 

Mercury Yellow or Turpeth Mineral is a basic mercuric 
sulphate of the formula Hg 3 S0 6 . It is obtained by heating 
10 parts of mercury with 15 parts of sulphuric acid in a 
porcelain dish in a good draught, until a white crystalline 
mass of neutral mercury sulphate remains. This salt, 
HgSO 4 , is decomposed in contact with water into free sul- 
phuric acid and a basic salt of the above composition. The 
decomposition is effected by treating the finely powdered 
neutral sulphate with hot water so long as the washings are 
acid, when a handsome lemon yellow substance remains. 
The wash waters contain acid mercuric sulphate. They are 
allowed to stand with mercuric oxide so long as this is dis- 
solved, and the solution then used to prepare new quantities 
of mercury yellow. 

Turpeth mineral has a very bright shade and great cover- 
ing power, but it has little permanence. Sunlight soon 
turns it grey, and air containing sulphuretted hydrogen in 
a short time turns it quite black, mercury sulphide being 
formed. 

Yellow Arsenic Pigments. The extremely poisonous 
character of arsenic pigments has practically banished these 
handsome and cheap colours from use. In many countries 
their use is justly illegal. The majority of arsenic pigments 
have, therefore, merely historic interest. The two yellow 
arsenic pigments are found in nature as realgar and orpi- 
ment ; though these are not rare minerals, the artificial 
products were generally used as pigments, and when they 



OTHER YELLOW PIGMENTS. 159 

were in common use they were generally made in metallur- 
gical works, in which minerals containing arsenic were 
treated. 

Kealgar, As 2 S 2 , has an orange red colour, whilst orpiment, 
As. 2 S 3 , is a pure yellow. When these substances were used 
as pigments they had the same drawbacks in regard to mixing 
with other pigments as other sulphur compounds. King's 
yellow is finely powdered natural or artificial orpiment. 

Lead Arsenite is a permanent deep yellow, but extremely 
poisonous. It can be made by fusing an intimate mixture 
of 100 parts of white arsenic with 75 parts of gold litharge, 
grinding and levigating the mass. Cadmium yellow, which 
has still more permanence and is less poisonous, replaces 
this pigment. 

Thallium Pigments. Thallium is a metal which exhibits 
certain similarities to lead. By precipitating a solution of 
:a thallium salt with potassium chromate or bichromate, 
according to the proportion between the quantities of the 
two salts, precipitates are obtained of yellow, orange or 
deep red colour, or, after fusion, brown. By the addition 
of a mixture of potassium chromate and ferricyanide to a 
mixture of a thallium salt and ferrous sulphate an olive green 
pigment is obtained. On account of the rarity of thallium 
compounds, technical employment is out of the question, 
and the sensitiveness of thallium pigments towards sulphur- 
etted hydrogen prevents their use for artistic purposes. 



CHAPTER XV. 

MOSAIC GOLD. 

MOSAIC GOLD consists of tin di sulphide, SnS 2 , fine scales of 
a golden yellow colour, which sublime undecomposed at a- 
fairly high temperature and withstand the action of chemical 
reagents. It has a peculiar greasy nature, and can be 
easily ground. It is, therefore, much used for bronzing 
picture frames, as a pigment for painters and for wall 
papers. 

Tin disulphide can be prepared either in the wet or the 
dry way ; in the wet way, by the action of sulphuretted 
hydrogen on a solution of tin tetrachloride. The yellow 
precipitate so obtained has no handsome colour. A far finer 
product is obtained in the dry way. The process is often 
regarded as accompanied by particular difficulties, but in 
reality it is quite simple. It is only necessary in preparing 
this pigment to take care not to raise the temperature above 
a certain point, otherwise a great portion of the tin disulphide 
will be decomposed into sulphur and tin monosulphide. To 
prevent the temperature from rising too high, an addition 
of ammonium chloride is made. This salt is volatile at a 
certain temperature ; heat which would otherwise raise the 
temperature above this point is used in volatilising the am- 
monium chloride. With a little care it is easy to interrupt 
the operation before all the ammonium chloride has been 
driven off. The mosaic gold then obtained has a real 
golden glitter. If the temperature rises too high, grey tin 



MOSAIC GOLD. 161 

monosulphide is formed, which naturally considerably dimin- 
ishes the brilliancy of the product. Ammonium chloride 
may be replaced by mercury or mercury compounds, which 
are volatile at a temperature below that at which mosaic 
gold is decomposed. When mercury compounds are used, 
both on account of their cost and poisonous nature the heat- 
ing must be conducted in glass retorts in order to recover 
the mercury. This operation requires great care if loss due 
to the breakage of the glass vessels at the high temperature 
is to be avoided. The process in which metallic mercury 
is used gives the finest product of all, and is to be recom- 
mended when a pigment is to be prepared which shall as 
nearly as possible approximate to the appearance of gold. 

In order to obviate the danger and loss associated with the 
use of glass vessels, manufacturers who make mosaic gold 
in large quantity should use an iron vessel. This will last a 
very long time. Such an apparatus consists of an iron pan 
with a broad rim, upon which is fastened a head which has 
the form of a retort neck ; to this are connected short, 
wide tubes leading to a chamber in which substances not 
condensed in the retort neck may deposit, so that the use 
of this inexpensive apparatus will not only be without danger, 
but will be accompanied by the recovery of almost the whole 
of the volatilised substances. The pan is filled with the 
materials, the head placed on, the joint tightly luted, and 
the retort neck connected with the chamber by the wide 
iron tubes. 

There are many formulae for the preparation of mosaic 
gold. Some of the most important are given, which in every 
case will yield a good result : 

Tin filings 40 parts 

Sulphur 35 

Ammonium chloride .... 25 

The tin filings, which must be very fine, are well mixed 

11 



162 MINERAL AND LAKE PIGMENTS. 

in a mortar with the sulphur and ammonium chloride. The 
heating is gradual at first ; when the evolution of vapours 
has ceased the temperature is very slowly increased to 
a dark red heat. The mosaic gold is found as a yellow mass 
at the bottom of the vessel, but partly in crystalline -scales 
on the walls and head of the retort. 
Other recipes are as follows : 

Tin dioxide 80 parts 

Sulphur 60 

Ammonium chloride . . . . 30 ,, 

Tin filings 45 

Sulphur 35 

Ammonium chloride . . . . 25 ,, 

In all these cases the chief endeavour should be not to 
raise the temperature too high ; a dark red heat is sufficient 
to give a perfectly satisfactory result. 

When metallic mercury is used, it is employed in the 
form of an amalgam with tin, which is then in such a finely 
divided condition that it readily enters into chemical com- 
bination with the sulphur. The amalgam is most simply 
obtained by heating 1 part of mercury almost to boiling, 
and stirring 2 parts of tin filings into the hot metal ; 18 parts 
of this amalgam mixed with 7 parts of sulphur and 6 of 
ammonium chloride are heated together. 

The mosaic gold made after any of these methods may 
be used for gilding gold frames or as a painter's pigment. 
Much so-called gold paint consists of mosaic gold ground 
with a thick gum solution. 

Chrysean, -Wallach found that when a current of sul- 
phuretted hydrogen was passed through a saturated solution 
of potassium cyanide a precipitate was formed, which had 
the formula C 4 H 5 N 3 S 3 . This substance, chrysean, is similar 
in appearance to mosaic gold ; its technical employment is 
prevented by its extremely poisonous nature and its high 
cost as compared with mosaic gold. 




Of THt 

UNIVERSITY 
or 



CHAPTEK XVI. 

RED MINERAL PIGMENTS. 

VERMILION. 

THIS beautiful scarlet red pigment, which has been used for 
so long a time, consists of mercuric sulphide, ! "HgS. The 
same compound occurs ready formed in nature as cinnabar ; 
picked pieces of this mineral come into the market under 
the name of mountain vermilion (Bergzinnober). A far larger 
quantity of vermilion is made artificially. 

Mercuric sulphide exists in two forms as a black non- 
crystalline powder and in the crystalline form, which is used 
as a pigment. Each modification may be transformed into 
the other by suitable treatment, and each may pass into the 
other spontaneously under certain conditions. In the manu- 
facture of vermilion, the black form of mercuric sulphide 
plays an important part ; it is, therefore, necessary to give 
an account of the chemical behaviour of the two modifica- 
tions before proceeding to an account of the method by which 
vermilion is made. 

Black Mercuric Sulphide may be obtained either by the 
direct union of metallic mercury with sulphur, or by pre- 
cipitating the solution of a mercuric salt with sulphuretted 
hydrogen. It is most simply formed by rubbing together 
equal parts of sulphur and mercury moistened with water, 
until the mixture is uniformly black. It is, however, difficult 
in this way to convert all the mercury into sulphide. A 
better result is obtained when the mixture is moistened by 



164 MINERAL AND LAKE PIGMENTS. 

ammonium sulphide instead of water. In this case the time 
required for the operation is shortened by warming the vessel. 
If the mortar is placed in hot water it is generally sufficient 
to grind for about two hours to bring about the combination 
of the mercury and sulphur. 

This compound can also be easily obtained by heating 
mercury with sulphur. In a vessel, placed under a chimney 
with a good draught, which is necessary to carry away the 
poisonous mercury vapours, 6 parts of the metal are heated 
nearly to boiling and 1 part of sulphur added. Combination 
takes place at this temperature with a slight explosion, and 
pure mercuric sulphide results when the heating is continued 
until the excess of sulphur is driven off. 

For the purpose of the manufacture of vermilion, black 
mercuric sulphide is most simply made by filling a thick- 
walled vessel with equal weights of mercury and sulphur, 
moistening the mixture with water and shaking or rotating 
the vessel for several hours. This can be done either in a 
rotating cylinder containing iron balls, or the vessel can be 
fastened to any rotating object to a water-wheel or to the 
fly-wheel of a steam-engine. The vessel in which the com- 
bination is effected should of course not be quite full. It has 
been found that a more jerky motion than that of rotation 
effects the combination of the mercury and sulphur in a 
shorter time. For example, an opportunity of fastening the 
vessel to a saw-mill would be of great advantage. 

The mercuric sulphide made by the above methods is a 
velvety black mass, which, even when exactly equivalent 
weights of sulphur and mercury have been used, is never 
quite pure. Carbon bisulphide will always extract a certain 
quantity of uncombined sulphur. The most important pro- 
perty of the black sulphide for the present purpose is that it 
is changed into the crystalline modification by heating to the 
temperature at which it volatilises. The sublimed mercuric 



EED MINEEAL. PIGMENTS. 165 

sulphide has the well-known fiery scarlet colour characteristic 
of vermilion. 

Red Mercuric Sulphide, or vermilion, exhibits, for a sulphur 
compound, considerable resistance to the action of chemical 
reagents ; dilute mineral acids do not decompose it. Un- 
fortunately, vermilion has another property which makes it 
quite unsuitable for the artist's use : in the course of time it 
gradually turns dull and at last is completely discoloured. 
This alteration of colour can only be ascribed to a return of 
the red crystalline modification into the black non-crystalline. 
When a white pigment is tinted by vermilion it should not 
be a lead pigment, or in a brief time it will turn black. A 
white pigment such as zinc white, which is not acted upon by 
sulphur compounds, should be used. 



CHAPTEE XVII. 

THE MANUFACTURE OF VERMILION. 

THE red modification of mercuric sulphide can be prepared 
in the wet or the dry way. The latter was formerly in 
general use, but at present the wet method is more gener- 
ally employed, as it more easily and certainly produces a 
handsome pigment. Each method has its advantages, and 
both will be described. 

(a) DRY METHOD. 

The numerous prescriptions which have been given for 
the manufacture of vermilion by the dry method are all 
founded on endeavours to convert black mercuric sulphide 
into the red form. Many of these prescriptions contain 
directions for the soundness of which no reason can be dis- 
covered, and it is not going too far to say that none exists, 
and that operations described as essential for the success 
of the process have been inserted merely to give the recipe 
the appearance of novelty. It cannot be denied that certain 
manipulations impart a greater brilliancy to the product, 
although it is impossible to assign a physical or chemical 
reason ; but the manufacturer will quickly be able to differ- 
entiate the valuable from the worthless in these processes. 
Two conditions have the greatest influence on the beauty of 
the pigment the temperature at which the black sulphide 
is sublimed, and the complete freedom of the vermilion from 
excess of sulphur. Of less importance is the repeated grind- 



THE MANUFACTURE OF VERMILION. 167 

ing of the vermilion; the oftener it is ground, the smaller 
the crystals become and the paler the shade. 

The operation of grinding the pigment in ordinary mills 
is known practically as " preparing," the extraction of excess 
of sulphur by boiling with alkaline liquids as "refining," the 
product. 

The usual process in Holland, especially in the Amsterdam 
works, is as follows : The black sulphide is made by heating 
270 parts of mercury with 37*5 parts of sulphur in copper pans 
the fire is so regulated that the temperature is not high 
enough to ignite the sulphur. If properly prepared the pro- 
duct has now a pure black colour. It is immediately finely 
ground and preserved in earthenware bottles containing only 
O7 kilogramme each. It is sublimed from hemispherical 
vessels, provided with iron covers carefully joined to the rim 
of the sublimation vessel by a suitable fire-resisting cement. 
Generally 3 such vessels are contained in one furnace. The 
operation is commenced by heating them until the bottom 
shows a dark red heat. The temperature should now be 
increased to such an extent that, when the contents of one 
of the above-mentioned small bottles are poured into the vessel, 
a small flame only appears ; if on the contrary the contents 
burn explosively, the temperature is too high, and further 
quantities of the black sulphide must not be added until the 
vessels have cooled down to a certain extent. If, on intro- 
ducing the first quantity of mercuric sulphide, a flame appears 
unaccompanied by an explosion, the contents of several of the 
bottles may be introduced ; the openings through which this 
addition is made are immediately closed by a well-fitting iron 
plate. From time to time this cover is raised and fresh 
quantities of mercuric sulphide added. The operation lasts 
about 36 hours, when double the quantity of mercuric 
sulphide, made from the above mixture of mercury and sul- 
phur, will have been introduced into the 3 vessels. For 



168 MINERAL AND LAKE PIGMENTS. 

the complete success of the process, the accurate regulation 
of the temperature to which the vessels are exposed is 
particularly important ; in practice, the temperature is judged 
by the height of the flame which issues on removing the iron 
plates : if this reaches to 1 metre the fire is too fierce, but 
if the flame is very small the fire must be increased. Towards 
the end of the sublimation the mass in the vessels is stirred 
about every 15 minutes. As soon as the sublimation is 
finished the fire is extinguished. The vessels are broken 
when completely cold ; the vermilion is then found in the 
upper portion as a sublimate of fibrous character. The 
vermilion made by this process requires simply wet grinding 
under ordinary mills and drying to be ready for market. 

In the great mercury works at Idria, in Austria, vermilion 
is also made from the black sulphide. The latter is made by 
mixing 84 parts of mercury with 16 parts of finely powdered 
sulphur in rotating vessels driven by water power, the opera- 
tion lasting about 3 hours. This quantity of sulphur is 
larger than is required to form mercuric sulphide ; experience 
has shown that the combination takes place more rapidly 
when more than the equivalent quantity of sulphur is used. 
Heat is developed by the reaction and the temperature of the 
mixture rises to over 30 C. The black sulphide is then 
sublimed in cast-iron vessels, which are pear-shaped and 
built 6 together in a furnace ; each holds a charge of 315 
kilogrammes of black. 

Several periods are distinguished during the heating of 
the mass in the sublimation vessels. The operation is com- 
menced by heating 2 of the vessels first. As soon as the 
vapours of sulphur, issuing from the neck of the vessels, take 
fire with explosion, the fire is made to heat the adjacent 
vessels. When the contents of all the vessels have exploded, 
the first part of the process known as the " evaporation " is at 
an end. Earthenware heads are then placed on the vessels and 



THE MANUFACTUEE OF VEBMILION. 169 

the fire is increased until the excess of sulphur present begins 
to distil ; its vapours take fire in the air with a slight ex- 
plosion. When this takes place, earthenware receivers are 
attached which have only a small opening for the escape of 
uncondensed vapours. The sulphur condenses in these. 
When sulphur vapours are no longer given off in quantity, 
the intermediate period (stuckperiode) is finished and the 
real sublimation of the vermilion commences. The fire is 
now considerably increased and the sublimed vermilion 
collects in the cooler parts of the apparatus. When the 
sublimation is completely finished, small blue flames, which 
quickly vanish, appear at the joints of the apparatus. The 
furnace is allowed to get quite cold, when the various parts 
of the apparatus are taken apart, the vermilion deposited in 
the tubes is carefully removed, so that they may be again 
used, whilst the receivers and head are broken, so that the 
vermilion they contain can be collected. 

The larger pieces form lump vermilion ; the fragments of 
the receivers are cleaned with a wet brush to collect what 
adheres. The whole process of sublimation from the intro- 
duction of the black to the end lasts about 7 hours. 

The sublimed vermilion is ground in mills which differ 
little from ordinary grinding mills. To prevent the formation 
of dust, water is added and the stones are surrounded by 
wooden casings. The red paste from the mills, which is 
now known as vermilion, is then refined. 

The refining consists in extracting the excess of sulphur 
by means of boiling potash solution ; 300 kilogrammes of the 
ground vermilion are stirred up with water in a tub, the water 
is drawn off and the wet mass brought into an iron pan, in 
which it is heated with 22 '5 kilogrammes of potash lye for 
about 10 minutes. According to the composition of the 
crude vermilion the lye has a strength of from 10 to 13 B. 
The smaller the quantity of sulphur, the weaker is the potash 



170 MINEEAL AND LAKE PIGMENTS. 

solution ; it is, or was, made in Idria in a very primitive 
manner, by extracting wood ashes. For vermilion of a bright 
red shade potash solution of 10, for the dark red of 11, and for 
" Chinese vermilion " of 13 is used. The excess of sulphur, 
together with a trace of mercuric sulphide, dissolves in the 
potash solution ; the sulphur chiefly forms potassium penta- 
sulphide. When the boiling is finished, the vermilion is 
carefully washed and dried in dishes placed in a heated 
furnace. During drying, the vermilion agglomerates ; finally 
the lumps are broken and sieved. 

Chinese Vermilion is in similar case to Indian ink. Both 
substances are in common use in Europe, they far surpass 
in quality our own manufacture, and in neither case do we 
know the exact method by which they are made. Genuine 
Chinese vermilion so far surpasses European in brightness 
that it is bought at five or six times the price. It is said on 
unauthenticated authority to be made by subliming a mixture 
of 4 parts of quicksilver with 1 part of sulphur in earthenware 
pots closed by an iron plate, which is kept constantly wet and 
serves as a receiver, on which the vermilion deposits. The 
sublimed masses adhering to the lid are sorted, ground and 
repeatedly washed with water. 

According to Callum's description of the manufacture of 
vermilion at Hong Kong, mercury and sulphur are heated in 
a large iron pan, with continual stirring at about the melting 
point of sulphur (111 C.), until the whole has changed to a 
black mass. After cooling, this is mixed with water and 
mercury, the mixture thoroughly stirred, dried, placed in a 
hemispherical dish and covered over with broken porcelain, 
A similar dish is cemented on the top of the first, and the 
dishes are heated for 16 hours ; the vermilion adhering 
to the porcelain fragments is removed, wet ground and dried. 

A vermilion approaching Chinese in beauty is said to be 
obtained by mixing ordinary vermilion with 1 per cent, of 



THE MANUFACTURE OF VERMILION. 171 

antimony sulphide and again subliming the mixture. The 
dark grey sublimate produces a reddish brown powder, which 
is repeatedly boiled with a solution of "liver of sulphur" 
(potassium polysulphide), washed with water and digested 
for a long time with hydrochloric acid. The author has 
repeatedly made vermilion by this process, but could never 
obtain a product differing appreciably from the original 
vermilion. 

(b) WET METHODS. 

The manufacture of vermilion in the wet way is founded 
on the conversion of metallic mercury or its compounds into 
mercuric sulphide by heating with alkaline sulphides, such 
as ammonium sulphide and liver of sulphur. The product 
always contains uncombined sulphur, which is eliminated by 
treatment with alkalis. 

Apart from the danger to the health of the workmen 
caused by the poisonous mercury vapours, which are present 
in rooms where mercury or its compounds are heated in 
apparatus even of the best construction, and which is not to 
be feared in a wet process, the vermilion prepared in the 
latter manner has a finer and brighter shade than that 
prepared by a dry process. The greater cost of the wet 
process is covered by the higher value of the product. 

The starting point of the wet method may be either 
metallic mercury, black mercuric sulphide or another mer- 
cury salt. When metallic mercury is used, according to an 
old process, 100 parts are ground with 23 parts of flowers of 
sulphur and a little caustic potash solution to a homogeneous 
mass, which is then heated with a solution of 53 parts of 
caustic potash in an equal quantity of water, the evaporated 
water being continually replaced, until the colour changes 
from brownish red to the fiery red of vermilion. When the 
colour approaches the desired shade a careful watch must be 



172 MINEEAL AND LAKE PIGMENTS. 

kept, and immediately the proper shade is obtained the heat- 
ing must be stopped. If it is continued beyond this point for 
but a short time, the vermilion at once loses its fire and 
cannot be again brightened. An interruption in the heating 
is equally harmful, a dull shade being produced. When the 
proper shade has appeared, the contents of the vessel are 
poured into a large quantity of water. The vermilion is 
then washed with dilute caustic potash and afterwards with 
water until the alkaline reaction disappears. It is finally 
dried. The alkaline solutions obtained in this process con- 
tain considerable quantities of mercury in solution. They 
are collected, and when a sufficient quantity has accumulated 
the mercury is extracted. 

Barff recommends the following method : Mercury is 
rubbed with ~ to i of its weight of sulphur until a uniform 
grey powder results. This is heated in a porcelain dish with 
caustic potash solution (133 parts of potash in 150 parts of 
water) at 45 C. until the powder has become bright red. 
Heating above 45 C. is to be avoided ; it would impart a 
brown tinge to the vermilion. 

A convenient process for making vermilion in the wet 
way is to first produce the black sulphide in the dry way 
and then treat this with alkalis. Brunner's method is 
founded on this procedure : 100 parts of mercury and 38 
parts of sulphur are used to prepare the black sulphide. 
This is heated with a solution of 25 parts of caustic potash 
in 150 parts of water. The operation is best conducted in 
a vessel placed in a water bath, the temperature of which 
does not exceed 45 C. After heating 7 to 8 hours, 
the mass begins to turn red, when the change proceeds 
more quickly, and the greatest care must be taken not to 
exceed the point at which the colour has reached its greatest 
brilliance. As soon as the desired shade has appeared, the 
temperature of the water bath is lowered, but it is kept warm 



THE MANUFACTUBE OF VERMILION. 173 

for several hours. Caustic solutions of different strengths 
produce different shades. Thus, in order always to obtain 
the same product from the same quantities, it is necessary 
to replace the evaporated water at short intervals. In this 
process some quantity of mercury remains uncombined ; it 
is separated from the vermilion by a process of levigation 
in the washing 

Firmenich's Method. Many wet methods for the manu- 
facture of vermilion differ but little from the preceding. 
In Firmenich's method the production of the black sul- 
phide is united with the formation of vermilion in one 
operation. 

A solution of potassium sulphide of a certain strength 
is made by igniting 20 parts of potassium sulphate with 6 
parts of coal, boiling the mixture with 1'5 time the 
quantity of rain water, cooling the solution, separating 
from the potassium sulphate which crystallises out, and 
boiling the solution with sulphur so long as it is dissolved. 
A mixture of 4'5 parts of this solution, 100 parts of mercury, 
and 2 parts of sulphur is placed in flasks, which are subjected 
for several hours to a shaking or rocking motion. The liquid 
becomes greenish, and its temperature rises in consequence 
of the combination of mercury with a portion of the sulphur 
of the potassium sulphide. The latter immediately dissolves 
the free sulphur present, and again gives it up to the mer- 
cury, which in the course of a few hours is completely con- 
verted into the black sulphide. The shaking is discontinued 
when the contents of the flasks have acquired a deep brownish 
red colour. The flasks are then placed in a room heated to 
45 C., and their contents repeatedly shaken up. At this 
temperature the transformation of the black into the red 
mercuric sulphide is completed. The deposit in the flasks 
acquires more and more a scarlet shade. As soon as the 
colour is found to have reached its greatest intensity the 



174 MINEBAL AND LAKE PIGMENTS. 

liquid is carefully poured off, and the vermilion treated with 
caustic soda solution in order to dissolve free sulphur ; it is 
then very carefully washed. 

In this process, the temperature at which the operation 
commences is important. The cooler the mixture which 
is placed in the flasks before shaking, the paler will be the 
the colour of the vermilion obtained. It is to be supposed 
that the reaction, which is but slow in the cold, forms in 
consequence of its slowness a black sulphide of such char- 
acter that in its subsequent transformation into the red 
crystalline sulphide very small crystals are produced. 

The Gautier-Bouchart method for the preparation of 
vermilion from mercury and ammonium sulphide is applied 
on the large scale as follows : 1,000 parts of mercury are 
shaken for 7 hours with 200 parts of flowers of sulphur 
and 400 parts of ammonium sulphide saturated with sulphur ; 
the dark coloured mixture is exposed for several days to a 
temperature of 60 C., when the colour changes to red. In 
addition to the usual washings, the mass is further treated 
with nitric acid. The purpose of this operation is to oxidise 
all the free sulphur to sulphuric acid. 

For a colour works in which, in addition to vermilion, are 
made other pigments, especially such as are sensitive to the 
action of sulphuretted hydrogen, this process, which pro- 
duces vermilion of a good shade, although not particularly 
stable, is not to be recommended on account of the evolu- 
tion of sulphuretted hydrogen, which cannot be completely 
prevented even by the greatest care. 

Liebig's Process. The starting point in the manufacture 
of vermilion by this process is the compound known as white 
precipitate (see below). It is only necessary to heat white 
precipitate with a solution of ammonium sulphide saturated 
with sulphur, at 40 to 50 C., for a long time. The opera- 
tion may be conducted in well-closed flasks in a place the 



THE MANUFACTURE OF VERMILION. 175 

temperature of which is, as nearly as possible, 45 to 50 C., 
such as the flues leading from a furnace in constant use. 
The change of colour is gradual ; this is an advantage, since 
it is easier to obtain the correct shade. By treating the pro- 
duct with weak caustic potash solution it may be made still 
brighter. After washing and drying at a gentle heat the 
pigment is finished. 

This method is especially adapted for manufacturers not 
exclusively engaged in making vermilion. No special arrange- 
ment of apparatus is necessary ; the apparatus for making 
ammonium sulphide and a number of glass flasks are the 
only essentials. There is another considerable advantage, 
that the product cannot be completely spoiled. The process 
of formation of the vermilion is tolerably slow ; a careful 
observation of the progress of the earlier operations is suffi- 
cient to determine the time required. In order to be able to 
do this with certainty, it is necessary to work always under 
exactly the same conditions ; the same quantity of white pre- 
cipitate must always be used and the solution of ammonium 
sulphide must always have the same strength. 

Mercuric Ammonium Chloride, Infusible White Precipitate, 
HgCINH 2 . This compound, which is required in the last- 
mentioned process for the manufacture of vermilion, is most 
cheaply made in the following way : to a solution of 1 part 
of common salt in 32 parts of water, 2 parts of dry mercuric 
sulphate are added in small quantities, whilst thoroughly 
stirring. It is absolutely necessary to work in this way, 
because mercuric sulphate is decomposed by water into free 
sulphuric acid and a basic salt, which is much more slowly 
converted into white precipitate than the neutral salt. The 
liquid now contains mercuric chloride ; on the addition of 
ammonia to alkaline reaction it gives a heavy white precipi- 
tate. The liquid is poured off and the precipitate washed 
with water containing a little ammonia, until the washings 



176 MINERAL AND LAKE PIGMENTS. 

give only a slight turbidity with barium chloride. This 
process may also be commenced with mercuric chloride, but 
the above method is cheaper. White precipitate should 
volatilise without fusing when heated on platinum foil 
and should keep its white colour when treated with am- 
monia. 

Electrolytic Process. In a wooden vessel 1 metre in 
diameter and 2 metres deep are placed, at the sides, dishes 
15 centimetres wide, containing a layer of mercury 1 centi- 
metre deep. These dishes are connected with the positive 
pole of a dynamo. The negative pole is connected to an iron 
plate, electrolytically coated by copper, placed at the bottom 
of the vessel, which is filled with a solution containing 8 
per cent, of ammonium nitrate and 8 per cent, of sodium 
nitrate. A regular current of sulphur dioxide is introduced 
through a perforated coil. The excess of gas escapes by 
a tube in the cover. When the current is passed a precipi- 
tate of red mercuric sulphide is at once formed. Attempts 
have been made to dispense with the current of sulphur 
dioxide The bath then contains 100 litres of water and 8 kilo- 
grammes each of ammonium nitrate, sodium nitrate, sodium 
sulphide and sulphur. Under these conditions it is only 
necessary to add sulphur and mercury in order to obtain, at 
the end of the operation, vermilion which is in no way inferior 
to that made by the first process. 

Vermilion is frequently grossly adulterated by cheaper 
pigments. Substances are often found under the name of 
vermilion which contain no trace of mercury, but consist of 
bright orange lead mixed with a few per cent, of ferric oxide, 
and having a deceptive similarity in colour to the best 
vermilion. 

Mercuric Iodide. When corrosive sublimate solution is 
precipitated with exactly the necessary quantity of potassium 
iodide, mercuric iodide is obtained as a scarlet precipitate 



THE MANUFACTUBE OF VERMILION. 177 

which surpasses in beauty even the best samples of vermilion. 
Unfortunately, this substance cannot be used as an artists' 
colour. Exposure to light soon turns it brown and finally 
black. It appears to be unaltered in the dark ; the author 
possesses a sample which has been so kept for 30 years 
without losing its shade in any way. 



12 



CHAPTEK XVIII. 

ANTIMONY VERMILION. 

ANTIMONY VEKMILION is a red pigment which will bear com- 
parison in fineness of shade with mercury vermilion, over 
which it has the advantage of cheapness. In composition it 
is antimony trisulphide, Sb 2 S 3 . This compound is obtained 
by precipitating a solution of antimony trichloride with sul- 
phuretted hydrogen. However, the precipitate, which is a 
very fine red whilst wet, loses its colour in drying, and the 
product is almost worthless as a pigment. 

In another way it can be obtained in such a condition 
that it loses nothing of its beauty in drying, but retains its 
brilliance. Bottger gives the following process : a solution 
of antimony trichloride is mixed with a solution of sodium 
hyposulphite (thiosulphate) and the liquid heated so long as 
a precipitate forms, which is then washed on a filter with 
water containing acetic acid. If pure water were used for 
washing, the antimony chloride still present would be de- 
composed, forming the white oxy chloride, which would 
detract from the shade of the antimony vermilion. In this' 
process particular regard is to be paid to the use of exact 
quantities of materials. The finest product is obtained when 
2 parts of a solution of antimony trichloride, which has 
exactly the specific gravity 1'35, are mixed with a solution 
of 3 parts of sodium hyposulphite in 6 parts of water. 

According to E. Wagner, antimony vermilion is obtained 
by dissolving 4 parts of tartar emetic and 3 parts of tartaric 



ANTIMONY VEKMILION. 179 

acid in 18 parts of water, heating to 60 C., mixing with a 
solution of sodium hyposulphite (thiosulphate) and heating 
to 90 C. The precipitate is then carefully washed and 
dried. 

Pure antimony vermilion closely approaches, as we have 
said, ordina^ vermilion in shade, and for a sulphur compound 
shows a remarkable resistance towards chemical reagents. By 
dilute acids, ammonia and alkaline carbonates, it is attacked 
only on long-continued contact, but it is easily decomposed 
by very dilute hydrochloric acid and by caustic alkalis. A 
mixture with white lead keeps for a long time, but there can 
be no question of the permanence of such a mixture, in 
consequence of the oft-repeated properties of lead pigments. 
Antimony vermilion is well adapted for oil painting. When 
ground with oil it exhibits a red of a brilliance in no way 
inferior to that of genuine vermilion. It may also be used 
as a water colour, but is not adapted for fresco work, since 
it is quickly decomposed by lime. 

In spite of these favourable properties antimony vermilion 
has so far been little used. Considering the small cost of its 
preparation, especially when calcium hyposulphite, which 
gives an equally good result, is used in place of the sodium 
salt, its use is to be recommended in the place of mercury 
vermilion. It appears as if the high price demanded for 
this substance by several manufacturers has prevented its 
general employment. 

Appendix, Antimony Blue. This fine blue pigment can 
be prepared by the addition of a dilute solution of potassium 
ferrocyanide to a clear solution of antimony in aqua regia. 
According to Krauss, it contains no antimony as colouring 
principle, but is a Prussian blue obtained from the ferro- 
cyanide, which is decomposed by the strong acid, with evolu- 
tion of hydrocyanic acid. 



CHAPTEE XIX. 

FERRIC OXIDE PIGMENTS. 

THE pigments composed of ferric oxide are used in enormous 
quantity. They are distinguished by a high degree of per- 
manence. Large deposits of ferric oxide occur in nature, 
and in places where it is found in considerable quantity iron 
is manufactured from it on the spot. Several varieties of 
natural ferric oxide are distinguished : specular iron ore 
forms crystalline masses of brilliant lustre ; another variety 
in small crystals is called iron glance ; micaceous iron ore 
consists of shining scales ; red haematite has the appearance 
of bundles of fibres ; an earthy variety of haematite is also 
common. 

The compound of ferric oxide with water, ferric hy- 
droxide, is still more abundant than haematite ; brown hae- 
matite, limonite and other iron ores consist essentially of this 
compound. The pigment known as ochre is also ferric 
hydrate. 

Very pure red haematite has so fine a red colour that it 
may often be used as a pigment after grinding or levigating. 
The famous red of Pompeii, which has been found on the 
ruined walls of the town, still shows, after eighteen centuries' 
exposure to damp, the brightest shade, certainly a striking 
proof of the extraordinary permanence of this pigment. 
Considering its great permanence, its easy preparation, and 
its low price, it is surprising that ferric oxide is not more 
extensively used by artists than it is at present. It is used, 



FERRIC OXIDE PIGMENTS. 181 

however, extensively in colouring earthenware, for which 
purpose its stability at high temperatures makes it suitable. 
Principally on account of its cheapness it is largely used in 
ordinary painting, but for artistic purposes it is not used to 
the extent it deserves. 

Every colour maker well knows that artists justly com- 
plain that the pigments offered to them have generally but 
a small degree of permanence. They are surprised that the 
paintings of the old masters show now, after the lapse of 
centuries, their colours unaltered, whilst the pigments manu- 
factured at the present day, instead of corresponding to the 
high standard of chemical knowledge, are often discoloured 
within a few months after use. But it was just the restricted 
knowledge of chemistry which the ancients possessed which 
compelled them to make extensive use of the permanent 
earth pigments, to which class of permanent colours ferric 
oxide belongs. The advances of science have succeeded in 
preparing ferric oxide, not only as a red pigment, but in the 
different shades of red, from yellow to brown and deep violet, all 
consisting entirely of pure ferric oxide. Ferric oxide has the 
property of altering its molecular condition on protracted 
heating ; this change is accompanied by an alteration in 
colour. If ferric oxide is heated for a very long time at 
the highest temperatures its colour changes at last to black. 
Ferric oxide can be prepared by different methods for 
artists' purposes. The process by which it is made is not 
unimportant. Either ferrous or ferric salts can be employed 
as the raw material. With the latter, pure. ferric oxide is 
at once formed, or hydroxide, which is converted into oxide 
by heating. The ferrous salts are generally cheaper than 
ferric salts ; they are therefore .commonly used for the pre- 
paration of ferric oxide, as well as of the other iron pigments. 
Even in combination with the strongest acids, ferrous oxide 
has but little stability ; when separated from its salts as 



182 MINEKAL AND LAKE PIGMENTS. 

ferrous hydroxide, the greatest precautions must be taken to 
obtain it pure ; in contact with the air it at once takes up 
oxygen and changes to ferric hydroxide. Ferrous carbonate 
shows this same degree of instability ; the natural substance, 
occurring in large crystals as spathic ironstone, is no excep- 
tion : on exposure to the air it is gradually changed to ferric 
oxide. 

There is another reason for the advisability of using 
ferrous salts to prepare ferric oxide. When ferric hydroxide 
is made by precipitating the solution of a ferric salt with 
ammonia or caustic potash, the least excess of the precipi- 
tant unites with the hydroxide to form a compound which 
is only decomposed by long washing with water. The 
precipitate is, however, gelatinous, and consequently very 
difficult to wash. 

In order to prepare ferric oxide suitable for an artists' 
pigment the following process may be used : 17 parts of 
soda are dissolved in 68 parts of water ; the solution is boiled 
in an iron pan, and 10 parts of crystallised ferrous sulphate 
are added in small quantities with continual stirring. The 
boiling and stirring are continued until the green vitriol has 
completely dissolved, when the greenish white precipitate 
is allowed to settle, washed several times with water, and 
then exposed to the air in thin layers. The precipitate, which 
begins to turn yellow during washing, becomes in a short 
time ochre yellow in the air, being changed into ferric 
hydroxide. After drying and calcining, a fine red powder 
of pure ferric oxide is formed. The shade depends on the 
temperature at which the substance is calcined : the higher 
the temperature and the longer the heating is continued, 
the darker is the product. 

Vogel's Iron Red. This preparation, which is particularly 
brilliant and therefore highly suitable for an artists' colour, 
is made by adding a saturated solution of oxalic acid to a 



FEEEIC OXIDE PIGMENTS. 183 

boiling solution of green vitriol. The greenish yellow pre- 
cipitate of ferrous oxalate is collected on a filter and well 
washed with water. After drying, the precipitate is heated 
in a shallow iron dish to a temperature of 200 C., at which 
the ferrous oxalate decomposes and is converted into a soft 
fiery red powder consisting of pure ferric oxide. By igniting 
this powder in covered crucibles the different shades of ferric 
oxide can be obtained. 

Macay's English Red, Seven hundred and four parts of 
ferrous sulphate, 1,000 parts of copper chloride, and 1,678 
parts of common salt are dissolved, the solution boiled and 
the precipitate ignited. 

In the manufacture of fuming or Nordhausen sulphuric 
acid, ferric oxide is obtained as a residue. It is then known 
under the names of English red, caput moriuum, colcothar, 
rouge and Indian red as a very cheap pigment. It is also 
used as a polishing material. Fuming sulphuric acid is made 
by heating green vitriol at a white heat in retorts placed in 
furnaces ; sulphur dioxide and trioxide are evolved, whilst in 
the retorts there remains a residue of almost pure ferric oxide, 
containing small quantities of basic ferric sulphate, which 
can only be decomposed by long continued violent heating. 
The vapours of sulphur trioxide are caught in receivers 
containing oil of vitriol, in which they dissolve and produce 
fuming sulphuric acid. 

The residue in the retorts, which has only a low com- 
mercial value, can be converted into a good pigment without 
the expenditure of much money or labour. It is ground in 
mills as finely as possible, and, if necessary, afterwards levi- 
gated. The fine powder is mixed with varying quantities of 
common salt, the object of which is to prevent the temper- 
ature from rising too high in the calcining process. Common 
salt is volatile at a temperature approaching a strong red 
heat ; when the temperature has once risen so far, a further 



184 MINERAL AND LAKE PIGMENTS. 

rise is prevented by the heat taken up in volatilising the salt. 
In order to make ferric oxide of a yellow tinge, 2 per cent, of 
salt are added, and the mixture heated with a moderate fire for 
1 hour. To obtain the deeper shades, the addition of common 
salt is increased even to 6 per cent. For a brownish red 
oxide, 4 per cent, of salt are added and the mixture heated 
for 4 hours ; for a dark violet oxide, 6 per cent, of salt 
are used, and the mixture is heated for 6 hours with the 
fiercest fire. 

It has been observed that the shade of the product is finer 
the more completely air is excluded from the glowing mass 
and the more quickly the product, after sufficient heating, is 
cooled down to the ordinary temperature.. 

Fire-clay tubes are used for heating the ferric oxide, which 
are similar to gas retorts, and are built one above the other 
in furnaces. The number in one furnace may reach 60. 
Each retort is closed by a well-fitting lid, which is luted with 
clay after the retorts are filled, a small opening being left 
through which the heated air may escape. If the retorts 
were closed completely air-tight they would burst on heating. 

Ferric oxide pigments are made in very large quantity by 
several works, which bring up to 20 different shades, varying 
from reddish yellow to dark violet, into the market. 

On account of their great resistance to the action of the 
atmosphere and of chemical agents, the ferric oxide pigments 
are particularly suitable for painting iron and other metals 
which are exposed to air or water. They are also suited to 
fresco work. 

Lower qualities of iron reds are made by calcining ochres, 
large deposits of which occur in nature, and also from the 
residues of basic ferric sulphate obtained in the alum manu- 
facture. These varieties are at the best usable for ordinary 
painting, but never for artists' pigments. 

According to the method of Steinau, iron pigments can 



FEEBIC OXIDE PIGMENTS. 185 

be made from wrought-iron scraps, turnings, etc., by causing 
them to rust through alternate contact with air and water. 
The resulting ferric hydrate is either at once used as a pig- 
ment or converted into other shades by calcining. By heat- 
ing in the air a red pigment is formed ; by heating with coal 
in the absence of air a black ; and from mixtures of red and 
black different shades of brown. If the iron turnings can be 
obtained cheap, this process should" be well adapted to the 
manufacture of fine iron pigments. 

Indian Red consists of ferric oxide. It was originally 
obtained from a very pure haematite, occurring in India, by 
grinding and levigating. It can, however, also be artificially 
made, in shades varying between bright red and dark brown- 
ish red, by heating pure ferric oxide. This valuable pigment 
is extremely brilliant and durable. 

The darkest brownish red shades can be obtained from 
Indian red by mixing it with varying quantities of litharge, 
and heating the mixture very strongly in a covered crucible. 



CHAPTEE XX. 

OTHER RED MINERAL PIGMENTS. 

Chrome Red or Chrome Vermilion, Under the most varied 
names, chrome orange, Persian red, Derby red, Chinese red, 
Indian red, chrome vermilion, etc., there occur in commerce 
numerous pigments, orange to dark red in colour, which can 
all be made from neutral lead chromate. When this salt, 
i.e., ordinary chrome yellow, is treated with smaller or larger 
quantities of a strong base, such as caustic potash or caustic 
soda, " basic " lead chromates are formed, which exhibit a 
more intense red shade the more lead they contain. These 
compounds can be made either by adding caustic potash to 
the solution of potassium chromate used for making the 
chrome, or the precipitated chrome yellow may be treated 
with caustic potash. The latter process readily yields good 
results, so that it is generally followed. 

Recent researches have shown that differences in the 
shade of chrome red are due to the varying size of the single 
microscopic crystals of which it consists. When chrome reds 
of the most different shades are ground the powder has al- 
most the same colour. The larger are the crystals the deeper 
is the shade of the red, so that the art of making deep chrome 
red lies in working so as to produce large crystals. 

As was said in describing the preparation of chrome yellow, 
the lead liquors used in its manufacture contain varying quan- 
tities of acetic acid, which affect the shade of the product. 
The quantities of caustic potash necessary to turn a certain 
quantity of chrome yellow into chrome red of a certain shade 



OTHEE BED MINERAL PIGMENTS. 187 

can, therefore, only be estimated from time to time in an 
empirical manner. This estimation may be performed by 
the method of Habich with great certainty. The precipitate 
of chrome yellow is made in the ordinary way, and well 
washed 6 to 8 times. Equal portions of the paste are 
brought into vessels of equal height and diameter. To the 
first of these portions caustic soda solution of a certain 
strength is added ; to the second rather stronger caustic 
soda solution ; in each succeeding vessel the strength of 
the solution is increased to a known extent. After the 
addition of caustic soda the vessels are energetically shaken. 
They are then set aside for the precipitate to settle in a place 
completely free from vibration. In one of the vessels, an 
orange or red shade will be seen very similar to that it is 
desired to make. Now, it is known how much caustic 
soda solution of a certain strength has been added to this 
quantity of colour, so that the quantity necessary for the 
whole of the chrome yellow precipitate may be found by a 
simple calculation. In working on the large scale, the process 
is exactly the same as in the preliminary test ; the calculated 
amount of caustic soda solution is added to the precipitate in 
the tub, it is well stirred, and left at rest for some hours. 
When the proper shade appears, the liquid above the pre- 
cipitate, which contains sodium chromate in proportion to 
the strength of the caustic soda solution used, can be drawn 
off and used to precipitate fresh lead acetate solution. The 
precipitate is washed first in the tub and then on the strainers. 

Since, as has been said, the depth of colour depends upon 
the size of the single crystals, care should be taken in wash- 
ing the precipitate not to agitate it too vigorously, or the 
pigment will lose in depth by the breakage of a large num- 
ber of crystals. 

Special formulae for the manufacture of chrome red are 
abundant, but if the above process be followed they are 



188 MINERAL AND LAKE PIGMENTS. 

superfluous, and it is only for the sake of completeness that 
one or two are given. Deep chrome red is made by the 
action of 25 parts of caustic soda on 100 parts of pure chrome 
yellow. 

A noteworthy process is due to Liebig. Saltpetre is melted 
in a crucible^ and heated to a temperature below that at which 
xygen is evolved. Dry chrome yellow is introduced so long 
as effervescence occurs ; the fused mass appears, whilst 
liquid, deep black. As soon as it is in quiet fusion it is 
poured on a cold plate. In consequence of the rapid cooling 
it becomes brittle and can be easily powdered. The hot 
mass is broken up and boiled with water, potassium chromate 
going into solution. The chrome red made by this process 
is little inferior in brilliance to vermilion. 

According to Prinveault a handsome chrome red is ob- 
tained by treating 25 grammes of neutral lead carbonate with 
10 grammes of yellow potassium chromate dissolved in 5 litres 
of water, boiling for half an hour, washing the violet pre- 
cipitate, and finally treating with 1 gramme of sulphuric acid 
diluted with 100 grammes of water. 

Cobalt Red consists of cobalt phosphate. It was first 
recommended by Salvetat. It is obtained by precipitating 
a solution of a cobalt salt by sodium phosphate. After dry- 
ing at the ordinary temperature the precipitate has a beau- 
tiful rose red colour. By careful heating, the shade becomes 
more violet according to the temperature used. 

Cobalt -Magnesia Red, When magnesia is moistened 
with the solution of a cobalt salt and strongly heated, a 
rose red mass is obtained which can be used as an extremely 
durable artists' colour. It probably consists of a compound 
of cobalt oxide with magnesia. The method of preparation 
is as follows : Magnesium carbonate is mixed to a thin paste 
with a dilute solution of cobalt nitrate ; this is heated and 
stirred until quite dry, when the residue is strongly heated 



OTHER BED MINERAL PIGMENTS. 189 

in a covered crucible. In the manufacture of this and other 
cobalt pigments it is of particular importance to prevent 
the entry of the fire gases into the crucible. Unless this is 
done a product of good colour is not obtained. 

Cobalt Arsenate. By precipitating a cobalt salt with 
sodium arsenate, a violet-red precipitate is obtained which 
leaves a fine durable red on heating. The mineral erythrite 
has a similar composition and colour. This pigment has a 
bright shade, is permanent, and although it contains arsenic 
is not particularly poisonous, since after fusion it is little 
soluble. 

Chromium Stannate. This artists' pigment, which may be 
used in oil and porcelain painting, is made in the following 
manner (Gentele) : 1 kilogramme of tin is converted by strong' 
nitric acid into metastannic acid, 50 grammes of potassium 
chromate are dissolved in 1 litre of water, the solution mixed 
with 2 kilogrammes of chalk and 1 kilogramme of powdered 
quartz, and finally with the metastannic acid. The mixture is 
dried and strongly heated. The temperature must be raised 
to a white heat, at which the mass sinters and acquires a 
dark rose-red colour. It is then completely extracted with 
boiling water, when a fine powder is left. 

This pigment, which is known in the market as " pink 
colour," may be used with advantage in oil painting in the 
place of rose madder lake, over which it has the advantage 
in durability. 

Silver Chromate. A solution of silver nitrate gives, with 
potassium chromate, a deep red precipitate of silver chrom- 
ate, which is occasionally used as an artists' pigment under 
the name of "purple red". Not only is the price of this 
pigment high, but it has little durability, since, like all 
silver compounds, it is acted upon by sulphuretted hydro- 
gen and turned dark, owing to the formation of black silver 
sulphide. 



CHAPTEE XXI. 

PURPLE OF CASSIUS. 

PUEPLE OF CASSIUS, or gold purple, is not a pigment for 
painters' use ; it is employed for colouring glass, for porcelain 
painting and for coloured glazes. For these last purposes 
we know nothing which can completely replace purple of 
Cassius, so that its preparation will be described here. Its 
costliness prevents its general use. 

As regards the chemical constitution of purple of Cassius 
very diverse views are held. Chemists have not yet suc- 
ceeded in explaining it ; some regard it as aurous stannate, 
others as stannic acid, in which a particular form of gold 
of a red colour is contained in an extremely finely divided 
condition. There are reasonable grounds for both opinions. 
Theoretical considerations are of little interest to the manu- 
facturer, who is chiefly interested in the method by which 
he can prepare a product which satisfies the demands of the 
consumer. 

Purple of Cassius has been known for nearly two hundred 
years, hence there are many formulae for its preparation. 
Before these are enumerated it is necessary to give an ac- 
count of the conditions under which this substance is formed. 
In order to obtain gold purple, a solution is required which 
contains both stannous and stannic chlorides ; if this solu- 
tion is mixed with a very dilute solution of gold chloride 
a precipitate is obtained which is generally brownish red, and 
only acquires its fine red shade on igniting. The colour 



PUKPLE OF CASSITJS. 191 

shown by the purple after precipitation is no guide to the 
shade of the finished product ; a purple which shows a very 
fine colour whilst wet often produces a pigment of much less 
beauty than an actually ugly precipitate. These differences 
are to be ascribed to the varying molecular condition of the 
purple, for the different varieties show very small differences 
in chemical composition. 

According to the directions of Fuchs, a fine purple is 
obtained by mixing a solution of stannous chloride with 
sufficient ferric chloride solution to give the mixture a green 
colour. Part of the ferric ^alt is then reduced to ferrous 
salt, whilst the stannous chloride is partially oxidised. At 
the same time a solution of gold chloride is made ; this must 
be quite free from nitric acid and contain 1 part of the salt 
in 400 to 500 parts of water. The tin solution is added drop 
by drop to the gold solution with constant stirring ; the 
mixture becomes turbid, but the precipitate requires a long 
time to settle on account of its fine state of division. The 
depth of colour shown by the purple varies according to the 
strength of the solutions. 

M. Miiller states that the following process gives the best 
results : The quantity of stannic chloride equivalent to 9 
grammes of stannic acid is dissolved in 200 cubic centimetres 
of water ; potassium carbonate is added to alkaline reaction, 
then 1 gramme of gold in the form of chloride. Grape sugar 
is added to the mixture, which is made up to 300 cubic 
centimetres by water, and warmed until the brightest shade 
is reached. When a very gelatinous mass is. formed after 
the addition of the potassium carbonate it is heated for a 
short time before adding the gold solution and grape sugar. 

Wachter gives directions for obtaining pale and deep 
shades. To obtain the pale purple 5 grammes of tin are dis- 
solved in aqua regia, the solution evaporated to dryness on 
the water bath, the solid residue mixed with a solution of 2 



192 MINERAL AND LAKE PIGMENTS. 

"grammes of stannous chloride and dissolved in 10 litres of 
water. To this liquid is added a solution of 0*5 gramme of 
gold in aqua regia, and immediately 50 grammes of ammonia 
to neutralise free acid. The precipitate separates sponta- 
neously from the dark red liquid ; its formation may be 
hastened by adding sulphuric acid. The washed precipitate 
is mixed whilst moist by means of a silver spatula with 20 
grammes of lead flux, 2 grammes of red lead, 1 gramme of 
quartz sand, and 1 gramme of calcined borax. The mixture 
is then dried ; it produces a purple red colour. If 3 grammes 
of silver carbonate are added, a clean, pale purple pigment 
results. 

The deep purple is made by mixing a solution of 0*5 
gramme of gold in 10 litres of water with 7*5 grammes of 
stannous chloride solution (specific gravity, 1*7) whilst stir- 
ring, and adding a few drops of sulphuric acid. The washed 
precipitate is mixed with 10 grammes of lead flux and 0'5 
gramme of silver carbonate. 

A rose-red shade of gold purple is obtained by dissolving 
1 gramme of gold and mixing the solution simultaneously with 
solutions of 50 grammes of alum in 20 litres of water, and of 
1'5 gramme of stannous chloride (specific gravity, 1*7). Am- 
monia is now added until all the alumina is precipitated. In 
order to prepare from the dried precipitate the mixture which 
will produce the colour on fusion, it is mixed with 50 grammes 
of lead flux and 2*5 grammes of silver carbonate. 

Magnesia Gold Purple, According to M. Miiller, the 
colour of gold purple is produced by covering the particles 
of a very finely divided white substance with metallic gold. 
Thus, when calcined magnesia is stirred up with water, gold 
chloride solution added, and the mixture warmed to 100 C., 
the gold is precipitated upon the magnesia ; the wet, yellow 
powder acquires a reddish shade on drying, which at a red 
heat becomes so beautiful a carmine-red that it surpasses 



PURPLE OF CASSIUS. 193 

purple of Cassius in fineness of shade. A purple which con- 
tains 20 grammes of gold as chloride to 84 grammes of mag- 
nesia has the most pure carmine tint ; the shade varies with 
the proportion of gold to magnesia. The different shades of 
purple contain the following percentages of gold : 

Percentage of Gold. Shade of the Purple. 

33-5 Brownish red (excess of gold). 

25-0 Deep carmine-red. 

20-0 Medium carmine-red. 

10-0 Pale carmine. 

5-0 A good rose-red. 

3-0 Eose-red. 

1-0 Pale rose. 

0-2 Delicate rose. 

Ol Appreciable red tint. 

Alumina Gold Purple is obtained, according to Miiller, 
by adding gold chloride and excess of potassium carbonate 
to alum solution and boiling. The purple obtained in this 
way is as deep in colour with 10 per cent, of gold as a 
magnesia purple with 20 per cent, of gold, but has a more 
bluish violet shade. If, in the preparation of purple, the 
alumina is precipitated by potassium carbonate and the gold 
chloride reduced by grape sugar, a different shade of purple 
is obtained to that in the precipitation of which ammonia is 
used. The alumina gold purple is specially adapted for the 
production of porcelain enamels. 



13 



CHAPTER XXII. 

BLUE MINERAL PIGMENTS IRON-CYANOGEN PIGMENTS. 

THE pigments occurring in commerce under the names of 
Prussian, Chinese, Berlin or Paris blue consist, when pure, 
of ferric ferrocyanide, Fe 4 [Fe(CN) 6 ] 3 . This compound is 
obtained as a deep blue precipitate when yellow prussiate 
(potassium ferrocyanide) is mixed with the solution of a 
ferric salt. 

The precipitate shows certain differences in composition 
according to the exact method by which it is obtained. 
A different result is obtained when the solution of the ferric 
salt is poured into that of a ferrocyanide, to that produced 
by the reverse procedure, the precipitate produced in the one 
case has properties which it does not possess in the other. 
Similar differences often occur in precipitating pigments. 

If the solution of the ferrocyanide is poured into the 
iron solution, the latter being in excess, an insoluble pre- 
cipitate is formed ;(but if the iron solution is poured into the 
ferrocyanide solution of which an excess is present, the blue 
precipitate is formed, but it is soluble in pure water, though 
not in water containing salts* Thus when this precipitate is 
.separated from the liquid and washed, the washings are at 
first quite colourless and remain so whilst salts are present, 
but when these have been completely washed away, a fine 
blue solution of the ferric ferrocyanide is formed, from which 
the addition of a salt solution again separates the dissolved 
Prussian blue. 



BLUE MINERAL PIGMENTS. 195 

Before proceeding to the manufacture of Prussian blue 
on the large scale, it is necessary to consider the behaviour 
of ferrous salts towards yellow prussiate, since these are 
commonly used instead of ferric salts. When a solution of 
potassium ferrocyanide is mixed with a solution of a ferrous 
salt, there is formed a pure white precipitate of potassium 
ferrous ferrocyanide, K 2 FeFe(CN) 6 . In order to obtain a 
completely white precipitate, it is necessary that the ferrous 
salt should be completely free from ferric salts and that the 
solutions should contain no dissolved oxygen ; the solutions 
would therefore have to be boiled before mixing. If one of 
the solutions contains the smallest quantity of oxygen 
a bluish precipitate is formed. When the white precipitate 
of potassium ferrous ferrocyanide is exposed to the air it 
immediately acquires at the surface a dark blue colour, and 
is completely changed to Prussian blue by sufficiently long 
contact with air. 

In accordance with this behaviour of ferrous and ferric 
salts different methods may be used to obtain Prussian blue. 
Either the process is commenced with a ferric salt from 
which a precipitate of Prussian blue is at once obtained, or 
the solution of a ferrous salt is converted into ferric salt by 
a powerful oxidising agent, such as chlorine or nitric acid, or 
a solution of a ferrous salt is precipitated with yellow 
prussiate and the white or bluish precipitate, which consists 
chiefly of potassium ferrous ferrocyanide, converted into 
ferric ferrocyanide or Prussian blue by treatment with nitric 
acid. 

Prussian blue is also known, as we have said, as Chinese 
blue, Berlin blue and Paris blue ; Brunswick blue or mineral 
blue is another form. Generally the product under the name 
of Chinese or Paris blue, which has an intense dark blue 
colour, is quite pure, whilst under the names of Prussian blue 
and Berlin blue mixtures with starch or alumina are included, 



196 MINERAL AND LAKE PIGMENTS. 

which consequently have a more or less pale colour. The 
pigments known as mineral or Brunswick blue are the least 
valuable, they have generally a less deep colour and are quite 
without the metallic lustre which distinguishes Chinese blue 
and the better varieties of Prussian blue. Mineral blue often 
contains very large quantities of alumina, chalk, or even 
barytes ; the addition of the last is to be regarded as irrational, 
since it makes the pigment conspicuously heavy. 

Chinese Blue. Pure Chinese blue appears in the form 
of deep blue masses, characterised by a peculiar metallic 
lustre, which is especially marked when the surface of a 
lump is rubbed with the finger nail. This metallic lustre 
is accompanied by a copper-red shimmer which is similar 
to that exhibited by fine indigo. In large pieces pure Chinese 
blue appears very dark by artificial light ; it possesses enor- 
mous strength of colour. The following test serves for the 
recognition of the complete purity of Chinese blue : A small 
quantity is powdered and rubbed in a thin layer on white 
paper ; if the metallic lustre shows in undiminished strength 
the blue may be regarded as quite pure, for Chinese blue 
quickly loses this property by additions of other substances. 

From what has been said above concerning the different 
behaviour of ferrous and ferric salts, the simplest method of 
making Chinese blue would be to precipitate the solution 
of a ferric salt with yellow prussiate, keeping the ferric salt 
in excess, to wash and dry the precipitate. Ferric salts are, 
however, dearer than ferrous salts, so that the latter are 
generally used. 

In making Chinese blue from a ferrous salt ferrous sul- 
phate or green vitriol is generally taken. This salt is dissolved 
in water ; it is advisable to add a small quantity of sulphuric 
acid, since the water will contain carbonates, which produce 
ferrous carbonate. Air oxidises the latter to ferric hydr- 
oxide, which is brown, and if present even in small quantity 



BLUE MINEEAL PIGMENTS. 197 

will spoil the shade of the blue. This addition of sulphuric 
acid is of special importance when the white precipitate pro- 
duced by the prussiate solution is to be oxidised by the air. 

A solution of yellow prussiate is added to the iron solu- 
tion in such quantity that a very small excess of iron salt 
is left ; there results a white or rather, since the ferrous 
sulphate always contains small quantities of ferric oxide, a 
pale blue precipitate. This is allowed to settle, the liquid 
drawn off as completely as possible from the precipitate, 
and nitric acid then added, or a solution of bleaching powder, 
followed by the amount of sulphuric acid required to decom- 
pose it. The nitric acid or the chlorine liberated from the 
bleaching powder speedily effects the oxidation, changing 
the pale blue colour to the deep blue of Chinese blue. The 
precipitate should be left for several days in contact with 
the liquid in order to complete the oxidation ; it is then well 
washed and dried. Pieces of definite shape and size bearing 
the trade mark of the firm are often pressed from the blue 
when it has acquired a stiff consistency. 

There are many formulae for the preparation of Chinese 
blue, differing in regard to the quantities of green vitriol, 
yellow prussiate, and nitric acid or bleaching powder to be 
used. It is, however, clear that only one formula can be 
correct, that in which the quantities of materials are equi- 
valent, since the reactions take place between equivalent 
quantities. In practice the equivalent quantities would not 
be weighed to the tenth of a gramme there would be no 
object in such a proceeding, since in works chemically 
pure substances are not used ; but manufacturers should 
work according to equivalent quantities in order to use up 
their materials as completely as possible. No increase in 
labour is involved, the same labour weighs one or another 
quantity. 

The proportion of materials in which the least loss is 



198 MINEKAL AND LAKE PIGMENTS. 

involved is here given. Nine kilogrammes of green vitriol 
are dissolved in 100 litres of water, 15 kilogrammes of strong 
sulphuric acid are added, and then a solution of 15 kilo- 
grammes of yellow prussiate in 100 litres of water. The 
solutions are kept in constant motion during the mixing. 
As soon as the decomposition is finished, without waiting 
for the precipitate to settle, steam is led in and 20 kilo- 
grammes of nitric acid of 1'3298 specific gravity added in 
small portions. The heating by direct steam is continued 
until red vapours are no longer evolved from the liquid, 
which is a sign that the oxidation is finished. 

Although the use of direct steam is very convenient for 
heating, since wooden vessels can be used and the heating 
quickly accomplished, it may be dispensed with, and the 
solutions heated in pans or even not at all. In this case the 
oxidation lasts considerably longer than when the liquid is 
heated. In the process recommended by Gentele 109 parts 
of yellow prussiate are used to 20 parts of green vitriol, each 
dissolved in much water. The precipitate is heated for a 
short time with 51 parts of nitric acid of T2285 specific 
gravity and 16 parts of sulphuric acid. The mixture is 
allowed to stand for several days to complete the oxidation 
before the precipitate is separated from the liquid. 

In Hochstatter's method, solutions of 6 parts of yellow 
prussiate in 15 parts of water and of 6 parts of green vitriol 
in 5 parts of water are mixed. To the precipitate 24 parts 
of strong hydrochloric acid and 1 part of sulphuric acid are 
added, and then a solution of bleaching powder in 80 parts 
of water until the liquid smells distinctly of chlorine. Apart 
from the fact that this method does not employ equivalent 
quantities of iron salt and yellow prussiate, it is also to be 
regarded as unsuitable, since the addition of sulphuric acid 
produces calcium sulphate, which, being soluble with great 
difficulty, will be precipitated along with the blue, making 



BLUE MINERAL PIGMENTS. 199 

the shade lighter. The addition of sulphuric acid should 
be omitted and only hydrochloric acid used, the calcium 
chloride formed by the decomposition of the bleaching 
powder being very soluble in water. Many formulae given 
for the manufacture of Chinese blue are said to produce a 
pigment of special beauty. This, however, is incorrect. 
Chinese blue is a compound of fixed composition, which is 
always formed when equivalent quantities of the salts are 
used and the precipitate converted into the blue compound 
by one of the oxidising agents given. It is most simple to 
use nitric acid as the oxidising agent ; for this purpose the 
presence of chlorine in the acid is harmless. When suffi- 
cient space and time are allowed, a special oxidising agent 
may be omitted and the oxidation of the white precipitate 
accomplished by atmospheric air. 

In working in this manner the precipitate is washed several 
times with water, and spread in a thin layer upon boards 
which are placed in rows one above the other. In a short 
time the mass turns blue on the surface, but the pale colour 
remains underneath much longer, since the access of oxygen 
to the interior is difficult. The boards must be left until 
the mass has a uniform colour throughout. In order that 
the paste shall not dry too thoroughly whilst on the boards 
it is desirable to make it very fluid. If the precipitate has 
been completely washed before the oxidation the product is 
finished after drying, but if a portion when completely dried 
shows a crystalline incrustation, salts are still present, which 
must be removed by careful washing. 

Prussian Blue. As we have already stated, Prussian blue 
is identical in chemical nature with Chinese blue, from 
which it differs only by certain additions which are made in 
order to reduce the price. Prussian blue should therefore 
be regarded as a more or less impure Chinese blue ; less 
pure materials are used in its preparation than for the fine 



200 MINEEAL AND LAKE PIGMENTS. 

Chinese blue. Instead of pure potassium ferrocyanide, the 
crude salt, which is sold by the prussiate makers at a lower 
price, is used for Prussian blue ; in addition to the ferro- 
cyanide this crude salt contains a considerable proportion 
of other salts, including a quantity of potassium carbonate, 

The ferrous sulphate used for Prussian blue is mixed 
with alum, which neutralises the potassium carbonate, pre- 
cipitating alumina, by which the precipitate formed from the 
yellow prussiate and ferric salt is made paler. If the pale 
blue precipitate were treated with acids they would first dis- 
solve out the alumina ; this should be avoided, since the alum 
has been added with the intention of increasing the quantity 
of precipitate and making the pigment paler. The precipi- 
tate can therefore only be changed into the blue pigment by 
spontaneous oxidation ; this is accomplished by the oxygen 
of the air in the manner previously described. The oxida- 
tion is more rapid according to the quantity of ferric salt 
contained in the green vitriol at the commencement ; it is 
convenient to prepare a large quantity of green vitriol solution 
and expose it in shallow vessels to the air so that the oxida- 
tion may proceed. But if the solution of green vitriol were 
exposed alone to the action of the air a rust-coloured pre- 
cipitate of basic salt would be deposited at the bottom of the 
vessels. To prevent the formation of this basic salt, sul- 
phuric acid is added in small quantity, and, as the oxidation 
proceeds, further small quantities of acid are added. In this 
way a solution of ferric sulphate is slowly formed, which 
gives with yellow prussiate a blue precipitate requiring little 
further oxidation. 

Mineral Blue, Brunswick Blue. This pigment, the most 
reduced of the colours of this class, is obtained by mixing 
white pigments, barytes, pipe-clay, etc., with the already 
impure Prussian blue. 

Soluble Prussian Blue. The modification of ferric ferro- 



BLUE MINERAL PIGMENTS. 201 

cyanide easily soluble in water is obtained by adding a 
solution of a ferric salt to a solution of yellow prussiate, the 
latter being in considerable excess. Briicke gives the follow- 
ing method : a solution is prepared of 217 grammes of yellow 
prussiate in 11 kilogrammes of water. Another solution 
contains 100 grammes of ferric chloride in 1 litre, 1 litre of 
which is mixed with 2 litres of a saturated solution of 
Glauber's salt and the mixture added to the prussiate solution 
so long as a blue precipitate forms. The Glauber's salt plays 
no part in the formation of the colour ; it is added to prevent 
the solution of the precipitate, which is insoluble in salt 
solutions. The precipitate is brought on to a filter and 
washed with water until the w r ashings begin to be blue, 
which is a sign that the salts have been almost completely 
removed. The precipitate cannot be further washed without 
the loss of a large quantity which would be dissolved by the 
water ; without further washing it is slowly dried in the air. 

Soluble Prussian blue has but a limited application ; 
anatomists use its solution to inject specimens, and it was 
formerly used for blue writing inks, for which purpose aniline 
dyes are now preferred. It may also be remarked that ordi- 
nary Prussian blue readily dissolves in a solution of oxalic 
acid ; the solution was also used as a writing ink, but since it 
attacks steel pens, it has now been replaced by solutions of 
dyes which have no action on steel. 

Special Processes for the Manufacture of Chinese Blue, 
In place of chlorine or nitric acid manganic chloride can be 
used to oxidise the white precipitate obtained from yellow 
prussiate and green vitriol. This process is based upon the 
property of manganic chloride to readily give up part of its 
chlorine and be changed into manganous chloride (the 
ordinary chloride of manganese). The conversion of the 
white precipitate into the blue is thus also in this case 
indirectly effected by chlorine. 



202 MINEBAL AND LAKE PIGMENTS. 

Many varieties of pyrolusite contain considerable quantities 
of manganic oxide which are of little use in the manufacture 
of chlorine, for which pyrolusite is chiefly employed. If such 
pyrolusite be digested with crude hydrochloric acid for several 
days, the manganic oxide is dissolved with the production of 
manganic chloride. 

The oxidation of the white precipitate is performed in 
the usual way ; after running off the liquid an excess of 
manganic chloride solution is added, as soon as two tests 
taken at an interval of a few minutes show no further 
increase in the depth of the colour, the reaction is ended and 
the solution of manganous chloride is drawn off. 

Any powerful oxidising agent may be used to oxidise the 
white precipitate ; chromic acid has been proposed for this 
purpose, but up to the present the chromates are too dear to 
be used in place of the cheap oxidising agents previously 
mentioned. 

In regard to the present endeavours to utilise by-products 
of the chemical industries, a method for preparing this 
pigment may be mentioned which would certainly be very 
profitable on the large scale. Coal gas before purification 
contains some quantity of ammonium cyanide, which is 
decomposed by ferric oxide with the production of Prussian 
blue. This pigment might be made by passing coal gas over 
very finely divided ferric oxide, until it was completely 
converted into Prussian blue. The process is, however, not 
practicable on the large scale : the ferric oxide would have to 
be exposed in an extremely thin layer to be completely con- 
verted into Prussian blue, and the transformation would 
require a long time. A better process is to make yellow 
prussiate instead of Prussian blue. A mixture of ferric oxide 
with coarse sawdust is spread out in thin layers, over which 
the impure coal gas passes. When the mass has acquired 
a blue colour owing to the Prussian blue produced, it is 



BLUE MINERAL PIGMENTS. 203 

treated with a mixture of quicklime and potassium sulphate. 
Potassium ferrocyanide is formed ; it is obtained in crystals 
by the evaporation of the solution and can be used in the- 
preparation of Prussian blue. 

The " Laming's mass," now generally used in gas works, 
contains, after use, a considerable quantity of cyanogen 
compounds, and it would only require a short trial to decide 
whether it was more advantageous to make this mixture by 
exposure to air again suitable for the purification of coal gas, 
or to use it to obtain yellow prussiate. 

TurnbulPs Blue. When a solution of a ferrous salt is 
precipitated by potassium ferrocyanide, a fine blue precipitate 
is formed, identical in physical properties with Chinese blue, 
but differing in chemical constitution. It is ferrous ferro- 
cyanide, Fe 3 [Fe(CN) 6 ] 2 . The manufacture of Turnbull's 
blue is expensive ; the pigment has no specially advantageous 
properties and is therefore seldom made. 

In preparing this pigment, a solution of yellow prussiate is 
transformed into red prussiate by passing chlorine through it 
so long as it is absorbed by the liquid ; the mother liquors from 
which red prussiate has been crystallised may be used with 
advantage. When to the solution of red prussiate green 
vitriol solution is added so that the former remains in excess r 
a soluble Turnbull's blue is obtained, but when excess of 
green vitriol is used the insoluble form is produced. 

Chinese blue and Turnbull's blue are used both as oil 
and water colours, and these handsome but not particularly 
durable pigments are also employed in colouring wall papers. 

Antwerp Blue is a mixture of Prussian blue with zinc 
cyanogen compounds. It is obtained by precipitating a 
solution of 2 parts of zinc sulphate and 1 to 2 parts of green 
vitriol (according as pale or deep blue is required) by a dilute 
solution of yellow prussiate. 



CHAPTEE XXIII. 

ULTRAMARINE. 

AT the present time blue, green, red and violet pigments 
come into the market under the name of ultramarine. The 
green and blue have been commercial articles for about 
seventy years ; the violet and red were introduced about the 
year 1860. 

At first the name ultramarine was restricted to a natural 
blue pigment obtained from lapis lazuli, which was extremely 
costly. Accounts of the payments of Italian artists, still ex- 
tant, show the expensive nature of the ultramarine blue used 
in their paintings. At that time, when artists were compelled 
themselves to make the majority of their pigments, ultra- 
marine was made in a most laborious manner from lapis lazuli, 
for which incredibly high prices were paid. In order to make 
the mineral easier to powder the lumps were heated, and, 
whilst hot, thrown into water. They were then powdered as 
finely as possible. The powder was mixed with melted resin, 
and the mixture kneaded under water for a long time. The 
ultramarine suspended in the water by this crude method of 
levigation was obtained by allowing the wash waters to 
settle. Few places are known at which lapis lazuli occurs in 
quantity. It is chiefly obtained in China and Thibet, and, 
considering the little intercourse between Europe and these 
distant countries at that time, it is no wonder that the price 
of ultramarine was fabulously high. One ounce cost about 
8 a price which is explained by the small yield of ultra- 
marine from the best lapis lazuli. By the most careful work 
not more than from 2 to 3 per cent, of the mineral was 
obtained, the residue consisted of foreign minerals. 



ULTEAMAEINE. 205 

The enormously high price of this pigment was the 
stimulus for the endeavours to make it artificially. The 
attempts are to be regarded not only as completely successful, 
but it must be allowed that science has gone a considerable 
step further than nature, since the researches have made it 
known that there is not only a blue ultramarine, but also a 
green, and, according to the latest researches, there exist in 
addition violet, red and white compounds, also to be 
described as ultramarines. With the discovery of the 
methods by which ultramarine can be made artificially, the 
preparation of this pigment from lapis lazuli came to an end, 
and has now but historic interest. The discovery of artificial 
ultramarine is due to the French chemist Guimet and to the 
great German chemist Gmelin. Ultramarine was first 
manufactured in Germany in the year 1828 by A. Kottig, as 
a branch of the porcelain works at Meissen in Saxony, where 
the manufacture was continued for about fifty years. 

The discovery of the aniline dyes is rightly called a 
triumph of human intellect. The artificial manufacture of 
ultramarine deserves the same description in no less degree, 
although it has not effected so great a revolution 'as the 
former. 

Attempts to make ultramarine artificially would naturally 
be based on the analysis of the natural product. The follow- 
ing comparison of the compositions of natural and artificial 
ultramarines shows how nearly the artificial product 
approaches the natural : 

NATURAL ULTRAMARINE. 

Clement and Desormes. Gmelin. 

Silica 35-8 per cent. 47'31 per cent. 

Alumina . ,34-8 22-00 



Soda 23-2 

Lime 3'1 

Sulphuric acid ... 

Sulphur 3-1 

Water and organic matter 



12-06 
1-55 
4-68 
0-19 

12-21 



206 



MINEEAL AND LAKE PIGMENTS. 



ARTIFICIAL ULTRAMARINE. 





Blue. 


Green. 


Observer. 


Warrentrapp. 


Elfen. 


Brunner. 


Pohl. 


Gentele. 


Silica 


45-60 


40-0 


32-54 


36-67 




Alumina 


23-31 


29-5 


25-25 


32-12 


47-31 


Soda 


21-47 


23-0 


16-91 


21-45 


39-93 












(sodium silicate) 


Potash 


1-75 











3-92 


Lime 


0-02 





2-38 





1-13 


Sulphuric Acid 


3-83 


3-4 





2-08 





Sulphur 


1-69 


4-1 


11-63 


7-22 


6-62 


Iron 


1-06 


1-0 


2-25 


trace 


1-95 












(ferric silicate) 


Water 

















Oxygen 








9-04 


0-58 






I. Szilasi found three samples of green ultramarine to 
have the following composition 

Water .... 2-20 1-20 1-19 

Aluminous residue . . 1-80 1-42 1-41 

Silica .... 16-73 17-18 16-74 

Aluminium . . . 15-92 15-87 16-15 

Sodium .... 18-42 18-18 18-10 

Sulphur .... 7-19 6'97 6'85 

There are many other analyses in addition to those we 
have given and agreeing with them, so that there is no doubt 
as to the composition of ultramarine, but as to the manner 
in which the elements are grouped nothing is definitely 
known. Some chemists are of the opinion that the colouring 
principle of ultramarine is a sulphur compound of iron, 
whilst others oppose this view and consider that the colour 
is due to the combination of a double silicate of alumina 
and soda with an unknown sulphide of sodium. Although 
no blue or green compound is known of corresponding 
composition, the majority of chemists incline to the latter 
view. Experience has shown that the presence of iron in any 
of the materials used in the manufacture of artificial ultra- 
marine is very dangerous to the success of the operation, and 
at the least considerably injures the beauty of the product. 

Although the manufacture of ultramarine is now very 



ULTEAMARINE. 207 

well known, it cannot be denied that some works produce a 
pigment of a shade which cannot be obtained by others. 
These works keep their method very secret, so that it is not 
possible to say with certainty whether they have introduced 
a process varying from that commonly known, or whether, 
by carefully watching the process, they have achieved great 
technical dexterity in the manufacture of this product. The 
latter appears the most probable, for in order to obtain 
a good result, many experiments, and an accurate knowledge 
of the raw materials, are necessary. 

The raw materials used are as follows : pure aluminium 
silicate, sodium sulphate, soda, sulphur, coal. The aluminium 
silicate is used in the form of fine china clay or kaolin, 
sodium sulphate and soda must be used in the anhydrous 
form, the sulphur is the ordinary commercial substance 
purified by distillation. Charcoal or coal containing little 
ash can be used. 

Whilst the remaining raw materials are always of similar 
composition, the china clay from different localities possesses 
a very varying composition. This substance must be carefully 
chosen. There is hardly any kaolin which is naturally of 
sufficient purity to be used without purification ; it is well 
known that the china clay used for porcelain is subjected to 
a thorough preparation before it is used. Kaolin, like all 
clays, has been produced by the decomposition of felspar ; 
when the aluminium silicate so formed was able to deposit 
without foreign admixtures, that mineral was formed which 
is the purest of all clays and is called kaolin. The more 
foreign substances are mixed with the aluminium silicate the 
further is the clay removed from kaolin. The impurities 
which generally accompany the aluminium silicate are 
quartz-sand, chalk and ferric oxide. We distinguish ac- 
cordingly between kaolin, white clay or pipe clay, clay, 
and lastly marl, a clay containing much chalk. 



208 MINEEAL AND LAKE PIGMENTS. 

Even the purest kaolin contains certain impurities, of 
which quartz- sand is the principal and the least harmful. 
Before kaolin can be used in the ultramarine manufacture it 
must always be purified by levigation ; it is then ignited at a 
low temperature and powdered under stamps or in mills. 
The other materials required are generally produced by the 
chemical works in a condition of such purity that they can 
be at once used. 

Occasionally Glauber's salt contains iron, which would 
spoil the shade of the ultramarine. The iron can be easily 
removed by dissolving the crystallised salt in water, adding 
a little milk of lime and leaving the liquid for several days, 
stirring frequently. The lime neutralises every trace of free 
acid, and at the same time produces an ochre-yellow pre- 
cipitate of ferric hydrate. The solution of Glauber's salt 
thus freed from iron is evaporated in reverberatory furnaces, 
in which the salt is then calcined. This operation is not 
only simpler than evaporating the solution in iron pans to 
crystallisation and subjecting the dried salt alone to calcina- 
tion, but it also guards against fresh contamination by iron, 
which might be caused by the use of iron evaporating pans. 

The sulphur and coal are employed in a soft powder, 
which is most readily obtained by placing the coarsely 
powdered materials in rotating drums containing a number 
of iron balls. By continued rotation the materials are con- 
verted to any desired degree of fineness without the produc- 
tion of dust. The powder is then put through fine sieves by 
which the larger particles are retained. 

The proportions in which the raw materials are mixed 
vary within certain limits ; fixed quantities can only be given 
for a kaolin of definite composition. Definite formulae are 
known by which ultramarine is made, but these can only be 
regarded as approximate. The proportions employed by 
French manufacturers differ considerably from those usual in 



ULTRAMAKINE. 209 

German works. This variation is chiefly due to the difference 
in the composition of the kaolin employed in the two 
countries. 

From the composition of the different mixtures one con- 
clusion may be drawn with certainty sufficient sodium is 
always used to neutralise half the silicic acid of the kaolin 
and to form some quantity of sodium sulphur compounds. 
In the successful process of the German makers a portion of 
the soda unites with the silica during heating. By the action 
of the coal on the Glauber's salt it is reduced to sodium 
sulphide, which, since sulphur is present, unites with a 
further quantity of that element. The sodium sulphur com- 
pounds then unite with the silicates of aluminium and sodium 
to form a green compound, which is converted into blue 
ultramarine by a further treatment with sulphur in the 
presence of air. 

Instead of using Glauber's salt, which must always be 
decomposed in the first process, the sodium sulphide may be 
formed by the action of sulphur on soda in the presence of 
coal. This procedure is adopted in the French process. The 
proportions of the mixture used in different works vary. If 
the kaolin employed is assumed to be bisilicate of alumina 
a somewhat arbitrary assumption the following mixture can 
be successfully used : 

Anhydrous kaolin 100 

,, Glauber's salt .... 42 

,, soda ...... 42 

Sulphur 60 

Coal 18 

In working by the French method the following formula 
is suitable : 

Anhydrous kaolin 100 

soda 100 

Sulphur 62 

Coal 14 

14 



210 MINEEAL AND LAKE PIGMENTS. 

These formulae are not to be regarded as unalterable. In 
the different works such varied mixtures are used that it 
may be said with truth that each works has its particular 
formula for the mixture, the composition of which depends 
on the nature of the clay used in the works. The composi- 
tion of the mixtures used in works is kept secret as far as 
possible. The formulae given above refer to a clay approxi- 
mating in composition to the bisilicate of alumina. 

It will appear from the description of the manufacture of 
ultramarine that certain quantities of sodium sulphide are 
produced. The process is, however, conducted so that sodium 
sulphide shall be formed ; thus the soda and Glauber's salt 
used in the mixture may be replaced by the sodium sulphide 
produced in previous operations. The liquors in which the 
sodium sulphide is contained are evaporated to dryness, and 
as much of the residue added to the clay as corresponds to 
the quantity obtained from the usual amounts of soda and 
Glauber's salt. Assuming that these latter materials are 
pure, 80 parts of the sodium sulphide residue correspond to 
100 parts of soda, and 60 parts to 100 parts of Glauber's salt. 



CHAPTEE XXIV. 

THE MANUFACTURE OF ULTRAMARINE. 

WE have thought it necessary to an understanding of the 
manufacture of ultramarine to discuss the production of this 
important pigment in some detail, for it is only possible to 
properly conduct an operation regulated by chemical laws 
when the processes which take place are accurately known. 

The mixture of the materials used in making ultramarine 
must be most intimate in order that the constituents may 
act chemically upon one another. In some works the 
mixing is performed in a very laborious manner upon a 
heap of the dried, levigated clay the remaining materials are 
thrown, and the whole shovelled about till it is completely 
homogeneous. Naturally, instead of this primitive and 
costly operation, mechanical mixers may be used, but the 
mixture can also be effected in a simpler manner, which 
almost dispenses with the necessity for a mechanical pro- 
cess. Of the substances to be mixed only kaolin and sulphur 
are insoluble in water ; the others and also the by-products of 
previous processes are easily soluble. The soluble and 
insoluble constituents of the charge may be mixed in a very 
simple way by bringing the levigated kaolin in the form of 
paste, without drying, into a pan, adding the solutions of the 
salts and the powdered sulphur. 

The pans are most conveniently heated by the fire gases 
from the ultramarine furnaces. After mixing the solutions 
with the clay to a thick paste the heating is commenced, the 



212 MINEEAL AND LAKE PIGMENTS. 

solid substances are prevented from sinking to the bottom 
by continual stirring, the mixture is slowly evaporated, and 
an extremely intimate and complete incorporation of the 
constituents occurs. The heating is continued until a dry 
mass is obtained, which may be used for the second opera- 
tion without powdering. 

The mixture of raw materials is heated for a long time 
at a clear red heat, or, in some works, at a white heat. The 
operation demands some care ; air must be excluded and the 
whole mass must acquire a uniform temperature. If both 
these conditions are not obtained in heating it is very difficult 
to obtain a product of uniform colour. 

In different works different arrangements are used for 
the calcining process. In the older methods crucibles or dishes 
of fire-clay were used, of such a shape that the bottom of one 
crucible formed the lid of the crucible beneath, just as is 
the case with the saggers used in porcelain kilns. The 
mixture is closely pressed into the crucibles, of which piles 
are built in the furnaces. These must be so arranged that 
each crucible is surrounded by the fire. The furnaces are 
very similiar to porcelain kilns. 

This method of calcining is obviously attended with many 
drawbacks ; a large number of costly crucibles is required, a 
certain proportion of which is lost at each burning, either 
by breakage in the fire or in filling. The pressing of the 
charge into the small crucibles requires much labour, as also 
the placing of the crucibles in the furnace and their removal ; 
at the end of the nine to ten hours' calcining, it is necessary 
to wait some time to take the crucibles out of the furnace, 
or considerable loss will be caused by breakage of the crucibles 
due to rapid cooling. 

For these reasons in most works crucibles have been 
abandoned in favour of muffles, a large number of which 
are built in one furnace. The charge is placed in the muffles ; 



THE MANUFACTURE OF ULTEAMAEINE. 213 

after completion of the calcining it is quickly raked out, and 
the still hot muffle at once recharged, thus the losses of time 
and heat are reduced to a minimum. The muffles are 
generally about 1 metre long, 1 metre wide and 30 to 40 
centimetres high. They are made of fire-clay, and, when 
kept in uninterrupted use, last a long time. The muffles 
have a small opening at the back which communicates with 
the furnace, so that the gases evolved have a free outlet 
without reaching the working place. The front of the muffle 
is closed by an iron plate in which are small openings, 
through which the contents can be examined without the 
entry of much air. 

Three muffles are generally built in a furnace, but, by 
suitable alterations in the construction, a considerably larger 
number may be heated in one furnace. Whatever the 
number of muffles, the furnace must be so arranged that the 
fire can be controlled at will. This is best done by a good 
damper ; all the muffles must be uniformly heated. 

J. Curtius recommends cast-iron cylinders, lined with 
a thin layer of fire-clay. For this purpose a covering of 
fire-proof cement is used to protect the retorts. This covering, 
as it is gradually destroyed, may be renewed by smearing over 
the damaged places with fire-proof cement, mixed with water 
or some binding material. Aluminium silicate, graphite and 
coke may also be used. The retorts, a (Fig. 28), project 
through the wall of the furnace at one end. They rest on 
fire-proof stone bridges, and can be put into communication 
with the air by the pipe, m, which can be closed air-tight. 
Short pipes, k and d, which can be closed, connect the in- 
terior of the retort with the cooling and collecting chambers, 
P and g, the latter of which is connected by r with the 
vessel, h, where the gaseous products are absorbed. The 
porous plate, /, prevents the charge in the hinder part of 
the retort from becoming open and falling to pieces during 



214 



MINEBAL AND LAKE PIGMENTS. 



stoking. In order to obtain a regular heating of the retorts 
the fire- gases passing through the space, q, are led away 
at two opposite places by flues into the main flue, F. The 
intimate mixture of finely-powdered materials is introduced 
into the hinder part of the retort, the porous plate, /, placed 
in position, the cover, t, closed up air-tight, arid the connection 
with the chamber, P, closed. On heating, the volatile pro- 
ducts pass through the porous plate and the pipe, d, into the 
chamber, g, the more volatile portions, without mixing with 
the furnace gases, passing direct to the absorption vessel, 
h, or to a lead chamber, whilst the sulphur which distils over 
remains in g. When the reaction is finished, by opening 




FIG. 28. 

the cover, t, and the tube, m, the crude green ultramarine 
in the retort can be rapidly oxidised to blue by aspirating 
air at m, or after closing the opening, d, and removing the 
plate,/, the green ultramarine can be raked into the collecting 
chamber, P, and there oxidised. The retorts are then im- 
mediately re-filled with the mixture and the temperature 
raised. 

The heating of a charge lasts nine to ten hours ; the 
larger the quantity of sodium sulphide in the mixture the 
shorter is the time required. The mixture js spread out in 
the muffles in a layer 7 to 9 centimetres thick, and brought 
to a moderate red heat. When the whole mass is uni- 



THE MANUFACTTJBE OF ULTBAMABINE. 215 

formly hot it is more strongly heated; the change of the 
constituents then becomes visible. The mixture takes 
at first a brownish colour, and is somewhat similar in 
appearance to liver of sulphur. The colour soon begins 
to incline to green, and finally changes to a tolerably pure 
green, but always has a yellow tinge. Those parts of the 
hot mass which come into contact with air are more blue, 
and occasionally change to a pure blue. When crucibles 
are used the upper layer of the contents is always bluer than 
the lower, and when crucibles crack during the heating the 
portions near the cracks always show distinctly a more or 
less blue colour. 

When the calcination is finished the hot mass drawn out 
of the muffles is at once brought into washing vessels, in 
which it is treated with water so long as soluble materials 
are extracted. The compounds dissolved in the water are 
chiefly sodium sulphide and sulphate and a little alumina 
dissolved in caustic soda. The presence of a larger quantity 
of caustic soda in the wash waters indicates too violent 
heating. In this case the mass will not mix up easily in 
water ; it contains sintered lumps. The washed material 
is strained, spread out on boards or linen stretched across 
frames, and completely dried by artificial heat. 

At this stage the product may be described as green 
ultramarine and sold as such, or it may be converted into 
blue ultramarine. For the latter purpose it is ground to 
a moderately fine meal, but green ultramarine must be 
ground to a very fine powder ; the finer the powder the 
brighter the shade. 

In order to obtain blue ultramarine from the washed and 
ground substance, it is subjected for a short time to a moderate 
red heat with unrestricted access of air. The heating by 
which the blue colour is produced must be continued until 
the proper shade is obtained. This operation is conducted 



216 MINEBAL AND LAKE PIGMENTS. 

in special muffles ; in many works it is customary to sprinkle 
powdered sulphur upon the heated mass during the roasting. 
The sulphur burns, forming sulphur dioxide, which escapes 
into the furnace through an opening at the back of the muffle. 
The object of the addition of sulphur may be to keep the 
temperature from rising beyond a certain point, or to prevent 
a possible reduction of the sulphur compounds in the ultra- 
marine. 

The muffles used for finishing the ultramarine are 
similar to those used in making green ultramarine ; 
they are generally 50 centimetres wide and 100 to 120 
centimetres long. A muffle of this size will contain 6 to 18 
kilogrammes of green ultramarine. The temperature is 
gradually raised to a gentle red heat, each muffle being 
provided in front with an iron plate which prevents the 
cooling of the contents by the external air. When the charge 
in the muffles is carefully observed, it is noticed that the 
blue colour first appears at the surface and the edges, i.e., 
at those places where the oxygen of the air has unre- 
stricted access. The mass is frequently turned over with an 
iron rake, in order to expose all parts to the action of the air ; 
samples are frequently taken in order to observe the moment 
at which the colour has reached its greatest intensity, when 
the heating is at once stopped. By proper treatment about 
half an hour is required to convert the charge of a muffle 
of the above dimensions into ultramarine of the deepest blue. 
The hot mass is drawn out of the muffles and spread out 
upon flags so that it may cool quickly. During the cooling 
the colour is often observed to become considerably darker, 
which is a sign that the ultramarine has not been heated 
for a sufficient length of time. 

Mechanical arrangements are often used to continually 
turn over the ultramarine in the muffles, which are then 
made in the form of cylinders in which the ultramarine is 



THE MANUFACTURE OF ULTRAMARINE. 217 

continually turned by a stirrer provided with wings. The 
whole arrangement is very similar to the apparatus used for 
roasting coffee. 

The ultramarine is then carefully ground to a very fine 
meal ; it is tolerably hard, and granite stones should be used, 
ordinary stones would be too quickly worn down. It is not 
possible, by the most careful grinding, to convert ultramarine 
into that condition of fine division which is necessary if it 
is to be ground into paint ; grinding must be followed by 
levigation. The various qualities and shades brought into 
the market differ only in regard to the fineness of their 
particles ; they do not vary in chemical nature, for the sub- 
stance has always the same composition. 

It occasionally happens in ultramarine works that a 
charge does not turn out a bright blue ; the product is then 
sold as inferior quality at a lower price, or is mixed with a 
larger quantity of good material. In working by a settled 
process the different shades of ultramarine vary in com- 
position between narrow limits ; the difference is caused by 
variations in the raw materials. 

It has been already remarked that there is no agreement 
as to the composition of the colouring principle of ultra- 
marine ; some maintain that it is an iron compound, whilst 
others regard the iron found in ultramarine as an accidental 
impurity, which has no connection with the colour. Eecently 
this point appears to have been decided from the results of 
many most accurate analyses of ultramarine. It has been 
found that its chemical composition shows the greatest 
similarity with the mineral nepheline. 

Nepheline is a double silicate, its composition is expressed 
by the following formula: Na 2 O . Si0 2 4- Al 2 3 .2Si0 2 . By 
a comparison of the analyses of green and blue ultramarine 
with that of nepheline, it is seen that blue ultramarine may 
be regarded as nepheline combined with sodium penta- 



218 MINEEAL AND LAKE PIGMENTS. 

sulphide, whilst green ultramarine is nepheline combined 
with sodium bisulphide. If this is the correct view, the two 
species of ultramarine have the following formulae : 

2{Na 2 O.Si0 2 + Al 2 3 .2Si0 2 } + Na 2 S 5 . 
2{Na 2 O.Si0 2 + Al 2 3 .2SiO 2 } + Na 2 S 2 . 

Ultramarine is one of the most permanent pigments. 
It is not altered by the substances with which a pigment 
generally comes in contact. Being a sulphur compound, it 
retains its colour completely in air containing sulphuretted 
hydrogen. By acids, even weak organic acids such as malic, 
citric or tartaric, it is rapidly decomposed, sulphuretted 
hydrogen being evolved, and a greyish-white residue left. It 
is sometimes found that the sugar used in making lemonade 
produces a perceptible smell of sulphuretted hydrogen. This 
is due to ultramarine, which has been added to the sugar to 
hide its yellow colour. 

From the preceding account of the manufacture of ultra- 
marine it will be understood that there exist only tw T o species 
of ultramarine, green and blue. In commerce a large 
number of different shades are found, which are not to be 
regarded as pure ultramarine ; they owe their particular 
shade to additions. By mixing in a white pigment, such 
as barytes or starch, the paler shades are obtained ; by 
adding a small quantity of a pure red pigment a colour 
with a violet tinge is produced. In a similar manner any 
number of shades of ultramarine can be obtained. 

The following account is taken from a new comprehen- 
sive work by J. Wunder, on the different ultramarines. 

Preparation of Mixtures for Ultramarine, The following 
points are to be observed : Chemically pure soda and ammonia 
soda react with difficulty. The presence of caustic soda in 
the soda facilitates the formation of sodium sulphide, and 
gives a finer product. It is also of advantage to sprinkle 
the soda with a strong solution of sodium sulphide. The 



THE MANUFACTUKE OF ULTRAMARINE. 219 

more silica the mixture contains the more difficult is the 
transformation into ultramarine, but the product is deeper 
and better in shade, and has more resistance to alum and 
weak acids. The sodium sulphide must react with silica 
and alumina. When oxygen enters during the burning, silica 
is re-formed, which with soda and alumina produces slags. 
In order to exclude oxygen many manufacturers burn with 
restricted access of air ; then the carbon bisulphide and 
sulphur gases evolved are not burnt, but, together with 
tarry materials from the coal, form an evil-smelling smoke,, 
which renders the neighbourhood objectionable. The coal 
consumption in burning is also greater. The varieties rich 
in silica and stable towards alum are generally dark reddish- 
blue. In order to obtain pure blue shades from mixtures 
rich in silica they are burnt in closed crucibles to produce 
the green, which is powdered and heated with restricted air 
supply, and the admission of a little steam, to 160 to 180 
C., when pure blue or greenish-blue shades are obtained 
as required. In this way a mixture rich in silica can be 
burnt to a good blue, containing 69'32 parts of silica to 
30*67 parts of alumina, or 1 equivalent of A1 2 3 to 3 '84 
equivalents of Si0 2 ; wiiilst ultramarine rich in silica, 
made in the ordinary way, contains at the most 66'7 parts 
of silica to 33'3 parts of alumina. 

Ultramarine Violet was first introduced into commerce 
about the year 1859 by the Nuremberg Ultramarine Works,, 
under the direction of Leykauf. By the reaction of moist 
calcium chloride on ultramarine blue in the warm chambers 
above the furnaces the blue was changed to violet. By the 
action of air a portion of the moist calcium chloride is 
decomposed into lime and hydrochloric acid, the latter of 
which, together with air, reacts on the blue. In 1872 Wunder,. 
intending to apply to the estimation of sulphur in ultra- 
marine blue a method which gives good results with metallic: 



220 MINEKAL AND LAKE PIGMENTS. 

sulphides, passed chlorine over heated ultramarine. The 
pigment was converted into a brownish-red substance, 
which on washing combined with water and turned violet. 
The red substance is not produced at every temperature ; 
300 C. is the most favourable. On washing sodium chloride 
is dissolved ; the washed violet is free from chlorine, but 
has taken up water, which is expelled on heating, when the 
violet turns to blue. A similar brownish-red body is formed 
when sulphur trioxide is led over heated ultramarine blue, 
and when sulphur chloride acts on warm ultramarine blue. 
When the brownish-red chlorine compound is introduced 
into absolute alcohol, a reddish-violet compound is produced, 
containing organic matter. Ethyl chloride is formed at the 
same time. When ammonia gas is passed over the heated 
chlorine compound it is absorbed ; a violet ultramarine is 
produced, from which the ammonia is not removed by 
washing with water, but only by heating nearly to redness, 
or, better, by fusing with caustic alkalis. Aniline combines 
in a similar manner. In order to obtain a good violet the 
brownish-red chlorine compound must not be formed ; the 
violet is produced by leading chlorine and steam over ultra- 
marine blue at 160 to 200 C. Blue rich in silica is most 
suitable for transformation into violet. The violet so obtained 
is not decomposed by lime. 

C. Mahla produced the chlorine in the blue itself by 
the reaction : NH 4 C1 + 2NH 4 N0 3 = 6H 2 + 5N + Cl. By 
heating a mixture of ultramarine blue with ammonium 
chloride and nitrate in crucibles at 200 C. a fine violet is 
produced. It contains ammonia, which cannot be removed by 
washing, but only by strongly heating or heating with con- 
centrated alkalis. This violet is decomposed by the prolonged 
action of lime, in three days it is changed to a grey green. 
In the course of the manufacture it was noticed that less 
nitrate was required in the mixture according to the length 






THE MANUFACTURE OF ULTRAMARINE. 221 

of time it was exposed in porous crucibles to the action of 
air and heat ; by heating for a sufficient length of time with 
access of air, a good violet is obtained with ammonium chlo- 
ride alone. In the manufacture of violet by means of moist 
chlorine hydrochloric acid is formed, which is also the active 
material in its formation by means of ammonium chloride. 
It was therefore to be expected that chlorine gas might be 
replaced by hydrochloric acid, if it were accompanied by 
sufficient air. This has been found to be the case. The 
temperature must be maintained between 180 and 230 C. 
Below 150 C. the blue is decomposed by moist hydrochloric 
acid gas ; at temperatures above 230 C. it is unchanged. 
Hydrochloric acid gas and air, without steam, give with 
ultramarine blue on heating a brownish red substance similar 
to that produced by dry chlorine ; it also is changed to violet 
by washing with water. The violet obtained by means of 
moist chlorine differs from the blue from which it was formed, 
in that the latter has lost one-sixth of its sodium and com- 
bined with water and much oxygen. The following is the 
approximate formula for a blue : 

Na 6 Al 4 Si 6 S 4 21 , 

and for the violet obtained from it 

Na 5 HAl 4 Si 6 S 4 24 + H 2 0. 

The violet contains much thiosulphate. If it is decomposed 
by nitric acid and silver nitrate added to the filtered solution 
a precipitate is obtained which changes in colour from white 
to yellow, orange and brown, just as the precipitate given 
by silver nitrate in solutions of thiosulphates containing 
nitric acid. 

Chlorine and Steam Process, Chlorine does not attack 
iron at 150 to 250 C. The reaction is carried out in heated 
iron boxes 1 metre wide, 2 metres long and 65 centimetres 
high. The blue is spread out in a layer 2 centimetres thick on 
earthenware plates, which stand one above another at a dis- 



222 MINEBAL AND LAKE PIGMENTS. 

tance of 5 centimetres, each supported on three feet. The 
plates are introduced by means of iron tongs through large 
openings in the top of the box ; after filling iron plates are 
screwed on to the openings, through them pass sheet-iron 
tubes reaching to the bottom, in which thermometers can be 
lowered on w T ires. The iron boxes stand in heated chambers, 
which have openings corresponding with the openings in the 
boxes, and shut off from the interior of the furnace. Chlorine 
and steam are led into the iron boxes at both ends through 
lead tubes reaching to the bottom ; the parts of these tubes in 
the furnace and the iron boxes are protected by wide sheet- 
iron tubes, the space between being filled with clay. The gases 
evolved, hydrochloric acid, sulphur chloride and steam, pass 
from the covers through earthenware pipes into boxes filled 
with limestone, upon which water drops, and thence to the 
chimney. After filling, the boxes are heated to 280 C. and 
steam introduced to remove sulphur ; they are then allowed 
to cool to 160 C., when chlorine and steam are led in for three 
hours. The violet is then finished, and the residual gases 
are blown out by a fan. 

Hydrochloric Acid and Air Process, The operation is 
conducted in the same iron boxes, upon the bottoms of which, 
beneath the openings, are earthenware dishes into which 
hydrochloric acid can be poured through earthenware pipes. 
The chimney draught must be strong enough to draw suffi- 
cient air through these pipes into the boxes. After the 
temperature has been maintained for seven hours at 220 to 
230 C., hydrochloric acid being poured in from time to time, 
the blue is changed to a dull violet, which becomes brighter 
when hydrochloric acid is repeatedly added at diminishing 
temperatures, 210, 200, 180 and 160 C. More recently the 
iron boxes have been replaced by stone chests or by chambers 
above the ultramarine furnaces, the temperature in this case 
being kept at about 200 C. 



THE MANUFACTUBE OF ULTRAMARINE. 223 

Ammonium Chloride Process. A mixture of ultramarine 
blue with 5 per cent, of ammonium chloride is heated during 
fourteen days in porous crucibles placed in the upper 
chambers of the ultramarine furnaces, when the contents of 
the crucibles become a handsome violet throughout. If 
sodium nitrate is used together with ammonium chloride, 
the violet is formed in a much shorter time. After washing, 
the violet still contains nitrogen ; on ignition or heating with 
strong alkalis it loses ammonia. Unfortunately, this fine 
violet is decomposed by the continued action of moist slaked 
lime. The violets made by the first two processes absorb 
ammonia when this gas is led over them at 180 to 200 C., 
and it cannot be removed by washing. 

Pale Blue Ultramarine. If the violet is heated in hydrogen 
at 280 to 290 C., it is converted into a pure, bright, pale blue. 
This has an absorption spectrum in which the red is not 
absorbed, but appears more brilliant than in the spectrum of 
ultramarine rich in alumina. Pale blue is turned violet blue 
by heating at 300 C., and at a red heat a dull blue. It is not 
yet made on a commercial scale, but on account of its great 
purity of shade it appears to be valuable for many purposes ; 
perhaps it may replace alumina cobalt blue. The composition 
of pale blue is : 

Calculated for Found. 

Na 5 12-4 per cent. 11-9 per cent. 

H 5 0-54 ,, 0-62 

A1 4 11-7 13-1 

Si 6 18-2 19-7 

S 4 13-9 12-7 

0^ 43-3 42-0 ,, (by difference). 

By a comparison of the composition of the violet and pale 
blue ultramarines, it is seen that the chief difference is an 
increase of hydrogen in the latter. 

Ultramarine Red. Since ultramarine violet increases in 
brightness and redness of shade in the air, Wunder errone- 
ously believed that this was due to oxidation, and that, 



224 MINEKAL AND LAKE PIGMENTS. 

consequently, the violet could be converted into a red by 
oxidising agents. Nitric acid vapours led over ultramarine 
violet at 170 to 200 C. do not act upon it, but where drops of 
nitric acid are spirted over, the violet is changed to red. 
Wunder then reduced the temperature to 135 to 145 C. and 
obtained the first ultramarine red. Iron is attacked by nitric 
acid at lower temperatures, but not at 135 C. ; the iron boxes 
previously described could therefore be used. It was after- 
wards found that at a sufficiently low temperature hydro- 
chloric acid gas converts ultramarine violet into red. The iron 
boxes cannot be used for this operation, as they are attacked 
at the temperature; the stone chests are used instead. Other 
acids also act on ultramarine violet ; boric acid gives a 
reddish violet. 

The violet is spread out on the dishes standing on three 
feet mentioned before, and heated to 128 to 132 C. At 
higher temperatures the violet is unaltered, whilst below 
100 C. it is decomposed. The hydrochloric acid is poured 
in from time to time through earthenware tubes into dishes 
in which it evaporates. 

A mixture of red and blue would appear violet, but would 
behave towards reagents in a different manner to real ultra- 
marine violet. Ultramarine blue is decomposed at 128 to 
132 C. by hydrochloric acid to a gelatinous mass, whilst at 
this temperature ultramarine violet is changed into a bright 
red. From the blue no violet can be obtained by nitric acid, 
but the violet gives a red at 135 to 145 C. Analysis 
would also indicate the difference. Ultramarine red has the 
following composition : 

Calculated for Found. 

Na 3 7'9 per cent. 8-1 per cent. 

H 5 0-57 0-72 

A1 4 12-3 13-3 

Si 6 19-1 ,, 19-3 

S 4 14-6 15-2 



Oo. 45-6 43-4 



'28 



(by difference). 



THE MANUFACTURE OF TJLTEAMABINE. 225 

It appears that in the red two more equivalents of sodium 
have been replaced by hydrogen. The violet is apparently a 
sodium salt of which the red is the acid. The violet made 
by means of ammonium chloride is also converted by hydro- 
chloric acid gas at 128 to 132 C. into a handsome red 
containing nitrogen, and the red is changed by hydrogen at 
280 to 290 C. into a lighter pale blue. 



15 



CHAPTEE XXV. 

BLUE COPPER PIGMENTS. 

Bremen Blue and Green. The pigment known as Bremen 
blue or Bremen green consists of copper hydroxide. It 
possesses all the disadvantages of copper compounds ; it 
is very sensitive towards sulphuretted hydrogen and sulphur 
dioxide; in contact with air containing the former gas it is 
quickly turned black. It has also other disadvantages ; it 
cannot be used in oil, for when ground in oil it is quickly 
discoloured, on account of the formation of copper oleate. 
If a wall which has not completely dried be covered with 
Bremen green, in a short time it will be covered with spots. 
In spite of this small stability, Bremen green is still largely 
used by artists on account of its low price, although it could 
be replaced by other pigments almost as cheap and consider- 
ably more durable. 

Copper hydroxide comes into the market under many 
names. The pigments known as blue verditer, lime blue 
and mountain blue contain essentially copper hydroxide. 
The preparation of pure copper hydroxide on the large scale 
will be first described. Copper sulphate is the usual raw 
material. In order to obtain a colour of a pure blue shade 
it must be free from iron, the presence of which would result 
in the production of a discoloured precipitate. 

A fairly concentrated solution of copper sulphate is 
warmed in a pan to about 30 C. ; weak caustic potash 
solution is added until the liquid is slightly alkaline. A 



BLUE COPPEE PIGMENTS. 227 

green precipitate separates, which consists of copper sulphate 
and hydroxide. In order to remove the copper sulphate from 
the precipitate, and impart to it the correct bluish-green 
colour, it is left on a strainer until it has acquired a pasty 
consistency, when it is brought into a tub and mixed with 
weak caustic potash solution. Care must be taken that this 
potash solution is not too concentrated, or it will withdraw 
water from the copp.er hydroxide and produce black copper 
oxide. In order to prevent this result, which would render 
the whole mass useless, a portion of the precipitate is tested 
with the caustic potash solution. If it is blackened, the 
solution must be diluted with the necessary quantity of 
water before it is used. Instead of diluting with water, 
potassium carbonate solution is often added. The addition 
of the caustic liquor is continued until the colour of a test 
portion of the precipitate is not deepened by further additions. 
When this point is reached the colour must be at once with- 
drawn from the action of the alkali. It is brought into a large 
tub rilled with water, in which it is most carefully washed 
with many waters. 

Basic copper chloride may also be used as the raw material 
for this pigment. It is obtained by treating scrap copper 
w r ith 60 per cent, of common salt and 30 per cent, of sulphuric 
acid. The pale green basic chloride is then treated with 
caustic potash solution. This method is now rarely used ; 
copper sulphate is considerably lower in price than formerly 
and copper hydroxide is prepared from it with the least trouble. 

Neuberg Blue is a mixture of copper blue with Chinese 
blue. The larger the quantity of the latter component the 
deeper the colour. Neuberg blue can be more easily ground 
with oil than pure copper blue. It should not be used as 
a distemper colour, for the Chinese blue is decomposed by 
the action of lime, and the colour will quickly be spoiled 
by the separated ferric oxide. 



228 MINERAL AND LAKE PIGMENTS. 

Lime Blue is ordinary copper blue, mixed with varying 
quantities of gypsum. It is paler than pure copper blue ; 
different shades are obtained by increasing the quantity of 
gypsum. 

This pigment is made by dissolving 15 parts of copper 
sulphate in 1,300 parts of water, adding a strong solution of 
121 parts of ammonium chloride, and then milk of lime 
prepared from 30 parts of quicklime. The precipitate is 
well washed, strained, ground, and finally dried. 

Lime blue is brought into the market in several forms ; 
in cubes obtained by cutting up the stiff paste and then 
completely drying, and in irregular lumps or a coarse powder. 
The pigments containing copper hydroxide as their colouring 
principle are used less as artists' colours than for distem- 
pering. 

Payen's Mountain Blue is a mixture of copper hydroxide 
with varying quantities of calcium carbonate. It is prepared 
by adding calcium chloride to a solution of copper sulphate 
and stirring in dilate milk of lime. A green precipitate 
of basic copper chloride is formed ; potassium carbonate 
solution mixed with milk of lime is then added. The latter 
mixture contains caustic potash, which decomposes the 
copper chloride, copper hydroxide and calcium carbonate 
being simultaneously precipitated. Different shades may 
be obtained ; by increasing the quantity of lime the product 
is paler. This pigment commonly comes into the market 
in the paste form. When it is dried the operation must 
be conducted at a low temperature ; copper hydroxide is 
converted into copper oxide at relatively low temperatures. 

Oil Blue. The pigment known under this name consists 
of copper sulphide (CuS). It can be made in several ways. 
The simplest process, which also gives a good product, is 
here described. 

Sulphur is boiled in a glass flask with a long neck. When 



BLUE COPPEE PIGMENTS. 229 

the heavy reddish-brown vapours begin to fill the neck, 
copper turnings or wire free from oxide are thrown in. The 
copper burns in the sulphur vapour with a red flame to 
cupric sulphide. When it has once commenced to burn, the 
flask requires little heating, so much heat is produced by the 
combustion of the copper that the sulphur continues to boil. 
The introduction of copper is continued until only a small 
excess of sulphur is left, the flask is then closed and allowed 
to cool. When the contents have reached the temperature 
of the air the flask is broken, and the mass boiled with 
caustic potash solution in order to remove excess of sulphur. 

The product obtained by this process has a handsome 
violet-blue colour, which must be shown by the whole mass. 
If portions taken out of the flask are black, an excess of 
copper has been used. In order to improve the colour of a 
faulty batch, it is mixed with sulphur and quickly heated to 
the boiling point of sulphur, air being excluded. The excess 
of sulphur is then removed by boiling with caustic potash 
solution. 

Oil blue is a handsome but not very durable pigment. 
When used in varnish, which protects it from the action of 
air, it is fairly permanent. 

Copper Hydroxide. By precipitating carefully a solution 
of a copper salt with caustic soda, a blue precipitate of copper 
hydroxide is obtained, which, though not distinguished by 
any particular beauty, is adapted for use as a distemper 
colour and for similar purposes. The operation is conducted 
at ordinary temperatures ; on account of its voluminous 
nature the pale blue precipitate requires a long time for 
washing. Hot solutions must not be used ; at a temperature 
near the boiling point of water copper hydroxide is completely 
decomposed into black copper oxide. 



CHAPTEK XXVI. 

BLUE COBALT PIGMENTS. 

Cobalt Blue, Thenard's Blue, Cobalt .Ultramarine, King's 
Blue, Leyden Blue, The pigment known under these names 
is a compound of cobalt oxide with alumina, and is prepared 
in a similar manner to Binmann's green. Of all the blue 
pigments used in painting, cobalt blue is the most handsome 
and most durable. It is completely unaltered by the atmo- 
sphere. The most powerful chemical reagents have but a 
slight action upon it, and their action is less the higher 
the temperature used in the manufacture. Cobalt blue 
is made by precipitating mixed solutions of a cobalt salt 
and alum, washing, drying, and igniting the precipitate. A 
product of pure blue colour is only obtained when alum 
absolutely free from iron is used. If but very small 
quantities of iron are present, the red ferric oxide has a very 
considerable influence on the shade of the product. 

The depth of colour varies according to the proportion of 
cobalt salt used. The variation is not so considerable as in 
the case of cobalt green ; as a rule, 50 to 100 parts of cobalt 
sulphate or nitrate are used to 100 parts of alum. The tem- 
perature at which the precipitate is ignited must be much 
higher than that used for cobalt green ; it should be raised 
to a white heat. The crucible should be provided with a 
well-fitting lid -the fire gases damage the shade of the colour. 

When cobalt sulphate is ignited with ammonia alum, 
cobalt blue is obtained ; but this method is not to be recom- 



BLUE COBALT PIGMENTS. 231 

mended. The ignition must be of long duration to com- 
pletely drive off the sulphuric acid. The author has made a 
cobalt blue of remarkable beauty in the following manner : 
Alumina obtained by precipitating electrolytically an alum 
solution (page 49) was mixed with a solution of pure cobalt 
chloride to a thin pulp, which was quickly dried in a shallow 
porcelain dish. The mixture, whilst still hot, was pressed 
into a crucible placed in a larger one filled with magnesia ; 
the crucible was heated to a good white heat, at which it 
was maintained for an hour, when it was allowed to cool. 
The cobalt blue made by this method has the purest blue 
colour, being without that admixture of red which is not 
rarely to be perceived in this pigment. To avoid the 
appearance of this red tint, certain additions are made ; 
a small quantity of a zinc salt is most frequently employed, 
according to the quantity of which more or less pale shades 
are produced. 

Cseruleum is a pale blue pigment consisting of a com- 
pound of the oxides of cobalt and tin. It is made by 
converting tin by the action of nitric acid into white stannic 
oxide, adding a solution of cobalt nitrate and evaporating to 
dryness in a porcelain dish whilst stirring ; whilst still warm 
the powder is strongly ignited. In cseruleum the chemical 
combination is less firm than in the compounds of cobalt 
oxide with zinc oxide or alumina, strong hydrochloric and 
dilute sulphuric acids decompose it, continued treatment 
with nitric acid extracts all the cobalt. Cseruleum resists 
completely all atmospheric influences. 

The circumstance that all the cobalt in this compound 
may be removed by treatment with nitric acid leads to the 
conclusion that the pigments composed of cobalt oxide and 
another oxide are not chemical compounds of the two oxides, 
but that the alumina, zinc oxide and tin oxide serve simply 
as bases for the blue modification of cobalt oxide. That a 



232 MINERAL AND LAKE PIGMENTS. 

chemical compound cannot be present follows from the fact 
that the quantities of the oxides uniting together may be 
very Considerably altered and a coloured product still 
obtained. The author has shown in 1868 that the blue 
modification of cobalt oxide is obtained by heating the red. 
The change of the red colour of cobalt chloride into the blue 
is not caused, as was formerly thought, by loss of water, but 
by the transformation of the cobalt oxide contained in the 
compound from the red into the blue modification. Strong 
bases are able to retain the cobalt oxide in the blue form, and 
strongly basic oxides are therefore used in the manufacture 
of this series of pigments. 

Cobalt Zinc Phosphate. This pigment is made by preci- 
pitating a solution of zinc sulphate, free from iron, by 
sodium phosphate, adding a solution of cobalt sulphate and 
again sodium phosphate so long as a precipitate is produced. 
Excess of sodium phosphate is to be avoided, otherwise a 
considerable quantity of cobalt remains unprecipitated. In 
the directions given by G-entele it is stated that the sodium 
phosphate must be iri excess, and that the liquid above the 
precipitate must possess a reddish colour. The author's 
experiments have shown that a pigment in every respect 
equal to that produced by Gentele's process is obtained 
when all the cobalt is precipitated. The precipitate, which 
consists of variable quantities of zinc and cobalt phosphates, 
is dried and ignited, when it acquires a very deep blue 
colour. 



CHAPTER XXVII. 

SMALTS. 

THIS blue pigment is a very finely powdered glass, coloured 
by cobalt oxide, according to the quantity of which products 
of very different shades are obtained. If a very small amount 
of cobalt oxide is used the smalts has a beautiful pale blue 
colour, but by the use of larger quantities an almost black 
mass can be obtained. 

Although the proportion of cobalt oxide contained in 
smalts is so variable, yet it is probably present as a compound 
of constant composition the cobalt potassium double silicate 
of the formula CoO . 2Si0 2 + K 2 . 2Si0 2 . The numerous 
published analyses of smalts lead to this conclusion. The 
small quantities of other metallic oxides which are found in 
smalts are to be regarded as chance impurities. Being a 
glass free from lime smalts is easily decomposed ; in this 
respect it shows great similarity with water-glass, which is 
completely soluble in boiling 'water. 

Smalts comes into commerce in the form of powder. In 
the " Blue Colour Works " in which smalts is made a number 
of qualities are distinguished ; these are separated according 
to the fineness of the powder. Smalts of good quality is 
composed of particles of the same size and has a pure sky- 
blue colour. A shade inclining to red is generally to be 
ascribed to the presence of iron ; material of this nature is 
of little value. 

Smalts is not easily attacked by chemical reagents. It has 



234 MINEEAL AND LAKE PIGMENTS. 

been above stated to belong to the easily decomposed glasses,, 
and it is attacked by reagents which ordinary glass completely 
resists. Finely ground smalts is strongly attacked by boiling 
sulphuric acid, and, after fusion with soda, is soluble in acids. 
Water has some action upon smalts ; by the long continued 
action of a large quantity the shade is injured, the colour 
acquires a greenish tinge. 

In the manufacture of smalts there are three principal 
operations : 

1. Preparation of the charge. 

2. .Fusion of the charge. 

3. Grinding the fused mass. 

These processes are principally mechanical. Only as 
regards the composition of the charge is chemical knowledge 
required. 

1. Preparation of the Charge, The cobalt is supplied by 
substances of different origin as a rule, roasted speiss cobalt 
and cobalt glance are used ; occasionally ores are employed 
without previous preparation. When pure cobalt salts (sul- 
phate and nitrate) are used, a product of very fine shade is 
produced, but in practice these salts are not applicable on 
account of their cost. There remain therefore only the com- 
pounds of cobalt occurring in nature. On account of the 
foreign substances they contain, these require a special treat- 
ment before they can be used in the manufacture of smalts ; 
the treatment varies according to the other elements present 
in the cobalt ores. Ores containing bismuth are first treated 
in order to obtain this valuable metal ; ores containing arsenic 
are roasted in furnaces of special construction, which are 
connected with a series of stone or wooden passages in order 
to condense the arsenic trioxide completely. 

If ores are employed which contain only a small quantity 
of cobalt, the same quantity of glass is melted time after 
time with fresh quantities of the poor ore. At each operation 



SMALTS. 235 

cobalt is taken up. In place of glass a mixture of potash 
and very fine quartz sand may be used with advantage. It 
produces that species of glass which most easily takes up 
cobalt oxide. The quartz sand is generally made by igniting 
flint and quenching in water to facilitate the grinding ; it is 
then powdered. River sand is generally so impure that it 
would produce a brown or green coloured glass, which, when 
fused with cobalt ores, would give smalts of a bad shade. 

2. Fusion of the Charge. This process is very similar 
to the fusion of ordinary glass. The materials, in very fine 
powder, are carefully mixed and then " fritted," that is, 
heated until the mass sinters together without melting. 
The real fusion is accomplished in glass furnaces. The 
fritted mass is fused in small " glass pots " made of fire-clay, 
in a furnace similar in principle to the ordinary glass furnace. 
The glass pots are made rather small, to hold about 50 
kilogrammes, in order to obtain as homogeneous a product 
as possible. They are placed to the number of six in a 
furnace heated by coal, in which a very high temperature 
can be obtained. This is essential, because the charge has- 
a high melting point and must be made quite fluid in order 
that impurities may settle. 

The furnace must be slowly heated at first to prevent 
the pots from cracking. When they are once heated the 
temperature is raised to fuse the charge, which is repeatedly 
stirred so that the residues may sink to the bottom. These 
residues consist chiefly of metallic sulphides and arsenides.. 
After six to seven hours' fusion the stirring is discontinued, 
and the temperature raised for a short time to the highest 
it is possible to attain, so that the smalts may be as fluid 
as possible. When this is the case it can be drawn out 
into long thin threads. The melted glass is then taken 
out and poured into cold water. The rapid cooling makes it 
extremely brittle, and considerably facilitates the powdering.. 



236 MINERAL AND LAKE PIGMENTS. 

When new pots are used for the fusion, their walls are 
considerably attacked by the alkalis contained in the glass. 
In order to prevent this a small quantity of common smalts 
is fused in them before the introduction of the real charge ; 
the inner surface then becomes covered with a protective 
glaze. 

3. Grinding the Fused Mass, The smalts is next 
powdered under stamps. These only differ from the ordin- 
ary construction in that both stamp and trough are made 
of granite. The coarse powder from the stamps then goes 
to mills of ordinary construction, in which it is ground as 
finely as possible in a current of water. A blue liquid runs 
from the mills into tubs. The coarser particles which settle 
in these vessels constitute the article known commercially 
as strewing smalts, which is of small value. The quantity of 
this quality is fairly considerable ; it cannot all be sold ; part is 
fused in a later operation with a fresh quantity of the fritted 
mixture. The liquid remaining in the tubs then gives the 
different qualities of smalts. After having deposited the 
coarsest particles in the first vessel it is brought into 
another, in which it stays only a few minutes, when it is 
run off into another vessel, in which also it remains but a 
short time. The liquid goes in this manner through from 
three to five vessels, and in the process deposits all the 
coarser particles. The oftener the liquid is brought into 
fresh vessels the finer are the particles still suspended in it, 
and the longer it should remain before being drawn off into 
another. 

When the liquid has run through three or four vessels, 
as the case may be, it still retains the finest particles in 
suspension. It is now run into a tank, in which it remains 
until it is perfectly clear and has deposited all the blue. 
The deposit in this tank has a dirty bluish-grey colour ; it 
is utilised by being added to a later fusion. It would be 



SMALTS. 237 

expected that this last deposit would have the best shade, 
since it consists of smalts in the finest state of division, 
which would no doubt be the case if the smalts remained 
unchanged. Smalts is, however, a glass which is rather 
easily decomposed. In grinding, the smalts comes in contact 
with large quantities of water, and in the succeeding levi- 
gation the water decomposes it ; the final residue is thus 
produced. In view of the easy decomposition of smalts 
by water, it is necessary to carry out all processes in which 
it is in contact with water as rapidly as possible, in order 
to obtain the largest yield of smalts. 

The smalts deposited in the successive vessels have a 
different degree of fineness, and are sold under particular 
marks. The Saxony works, which produce the greater part 
of the smalts used in Germany, distinguish the qualities by 
letters. The better qualities are distinguished by the same 
letter, the fineness of the quality being indicated by the 
repetition of this letter. Thus FFFE, FFE, and FE in- 
dicate qualities which are of lower value the less often the 
letter F occurs in the mark. The qualities following those 
marked F are distinguished by M ; the lowest quality is 
marked OE. F indicates fine ; M, medium ; and 0, ordinary. 

The qualities generally designated smalts are those de- 
posited in the third and fourth vessels ; they are also known 
as king's blue or azure blue. 

The processes used in different works apparently vary. 
The essentials of the processes are, however, as they have 
just been described ; the time during which the liquid is 
left in the various depositing vessels is different. 

Smalts, being a glass, well resists the action of the 
weather, although on account of the absence of lime it 
has not the great chemical indifference of ordinary glass. 
Smalts is not altered by atmospheric influences, but from 
analogy with the behaviour of water glass, to which smalts 



238 MINERAL AND LAKE PIGMENTS. 

is closely related in chemical composition, we should con- 
clude that, although smalts resists the action of the air for 
a tolerably long time, it cannot be regarded as absolutely 
permanent ; in the course of time potassium carbonate and 
silica will be formed. 

Since the introduction of the manufacture of ultramarine, 
the use of smalts has considerably decreased. Ultramarine 
is incomparably the more handsome pigment, but it cannot 
he exposed to high temperatures without undergoing change 
of shade. For purposes in which the pigment is subjected 
to a high temperature, as in porcelain painting or earthware 
glazing, smalts has maintained its position. Smalts is also 
used in fresco painting, for colouring wax and tinting 
writing paper. For the latter purpose it should be rejected. 
It has no influence on the durability of the paper, but it 
spoils steel pens. Any one who has written for a long time 
on paper coloured with smalts must have observed this ; the 
best steel pens are soon worn out by such paper. The very 
fine splinters of glass present in large number in the paper 
act upon the steel as a very fine but very hard file, and 
quickly wear it down. 

When smalts are used for painting earthenware, the out- 
lines of the design are quite obliterated, the cause of which 
is that smalts, being itself a glass, fuses with the glaze (also a 
glass) of the vessel. When clean outlines are required 
smalts should not be used ; some other cobalt pigment resist- 
ing heat should be substituted. 

In addition to the above blue pigments which are in 
common use, there are two others to be mentioned, rarely 
used and very costly tungsten blue and molybdenum blue. 

Tungsten Blue is made by adding excess of ammonium 
chloride solution to a hot solution of potassium tungstate, 
collecting the precipitate resulting when the liquid has com- 
pletely cooled, drying and heating to redness in a crucible, 



SMALTS. 239 

through the cover of which a porcelain tube reaches to the 
bottom. When the crucible begins to glow, hydrogen is 
passed through the tube ; the crucible is then heated to a full 
red heat for fifteen minutes, the current of hydrogen being 
continued. The pigment obtained in this way is a deep blue 
powder of velvety appearance. 

Tessie du Motay's Blue. The following is the method of 
preparation : 10 parts of sodium tungstate, 8 parts of tin 
crystals, 5 parts of yellow prussiate, and 1 part of ferric 
chloride are dissolved separately and the solutions mixed. 
The precipitate is washed and exposed to light in thin layers. 
The blue colour develops in the course of several days. 
According to the discoverer, this pigment consists of a com- 
pound of tungsten oxide with a double cyanide of iron and 
tin ; in physical properties it is similar to good Prussian 
blue, but is distinguished by greater durability on exposure to 
light and also by a much higher cost. Up to the present 
this pigment has not been made commercially. 

Molybdenum Blue. This fine but costly pigment is 
obtained by mixing a solution of sodium molybdate with a 
solution of stannous chloride. A blue precipitate is formed, 
which, after washing and drying, may be used as an artists' 
colour. It is distinguished by great durability. A good 
shade of molybdenum blue can be obtained with greater 
certainty by adding finely-powdered tin and a few drops of 
hydrochloric acid to a solution of pure rnolybdic acid. In 
composition it is a mixture of stannic molybdate and the 
blue modification of molybdenum oxide. 



CHAPTEE XXVIII. 

GREEN MINERAL PIGMENTS GREEN COPPER PIGMENTS. 

Copper Carbonate. In nature two salts occur containing 
copper, carbonic acid, and water in different proportions 
one, the mineral malachite, has a fine emerald green colour ; 
the other, azurite, is sky blue. Both minerals are ground 
and levigated, and brought into commerce as mountain green 
and mountain blue. 

Compounds of copper with carbonic acid, made artificially, 
are used under different names as pigments. The terms 
mountain blue and mineral blue are applied indiscrimi- 
nately to various copper compounds ; as mountain blue, or 
Brunswick green, pigments come into commerce containing 
varying quantities of copper carbonate (made by precipitating 
a copper salt with soda) mixed with barytes, zinc white, or 
another white pigment, in order to obtain paler and deeper 
shades. 

The poisonous Scheele's green is also given the same 
name as that under which copper carbonate is sold, viz., 
mineral green, so that the composition of this colour can 
only be decided by analysis. 

A green pigment, similar in composition to malachite, is 
obtained by precipitating copper sulphate with soda in the 
cold and washing quickly. If the precipitate is left in 
contact with the liquid, its colour changes and it is converted 
into a blue mass similar in properties to ground malachite. 
A green precipitate is obtained with more certainty by pre- 



GREEN MINERAL PIGMENTS. 241 

cipitating a boiling solution of copper sulphate with soda 
solution. 

Both green and blue copper carbonate are pigments of 
little depth ; as a rule ground and levigated malachite and 
azurite have a greater intensity of colour than the artificial 
pigments. 

Copper Arsenite The compounds of arsenious acid with 
copper are among the most handsome and bright colours 
which exist. It is thus to be regretted that these pigments 
are disappearing more and more, and indeed with every 
reason, from the pigments permitted by law. In the end 
they must fall into complete disuse. It is a matter of 
common report that these pigments, and particularly emerald 
green, have frequently caused arsenical poisoning. Numbers 
of people have been repeatedly made ill by the dust of 
emerald green fixed on light fabrics, such as are used for ball 
dresses, simply by albumin or dextrine, and brought into 
the air by the movements of the dancers. It has been 
further shown that the air of rooms papered with hangings 
coloured by emerald green contains small quantities of the 
most poisonous of all arsenic compounds, arseniuretted 
hydrogen. 

Although it is in the interest of colour makers to produce 
the brightest possible colours, yet, on account of their dan- 
gerous properties, pigments containing arsenic should be quite 
excluded. In several countries their use is forbidden by law. 
Since there are several pigments which can compete in bright- 
ness with the arsenic colours, it is to be hoped that the latter 
will shortly disappear from commerce. 

Scheele's Green, Swedish Green, A solution of 32 parts 
of potash is boiled with 11 parts of white arsenic (arsenious 
acid) until the latter is completely dissolved. This solution 
of potassium arsenite is mixed with a hot solution of 32 

parts of copper sulphate free from iron. The beautiful green 

16 



242 MINERAL AND LAKE PIGMENTS. 

precipitate is washed with hot water and dried. These 
directions for the preparation of copper arsenite are due to 
Scheele, the discoverer of the pigment. It can be prepared 
by several other methods. 

When white arsenic and copper sulphate are dissolved 
together, and caustic soda added to the boiling solution, 
Scheele's green is precipitated. When made by this method 
the pigment must be very carefully washed ; if the excess 
of caustic soda is not completely removed it is very hard 
when dry, and is difficult to powder. In order to obtain 
darker greens the quantity of copper sulphate is increased. 

A particularly bright green is obtained by dissolving 
copper sulphate in water, with the addition of 12 to 15 
per cent, of white arsenic, and precipitating by a solution 
of potassium zincate, which is obtained by boiling caustic 
potash solution with zinc filings, hydrogen being evolved. 

The shade of Scheele's green depends on the relative 
quantities of arsenious acid and copper sulphate employed. 
If the arsenious acid predominates, a pale product is ob- 
tained ; when excess of copper sulphate is used, the colour 
is pure green. The pigment made according to the foregoing 
directions possesses only small covering power. A product 
better in this respect is obtained by precipitating a solution 
of 100 parts of copper sulphate by a solution of 90 parts 
of potassium carbonate, in which 66 parts of arsenious acid 
have been dissolved. 

Scheele's green is a bright green pigment, which resists 
the action of the atmosphere tolerably well. It is easily 
decomposed by dilute acids ; when strongly heated arsenious 
acid is set free, and copper arsenate and arsenide are pro- 
duced. Copper arsenite is the essential constituent of the 
pigments sold under the name of mineral green, which is 
generally a mixture of copper arsenite'with varying quantities 
of mountain green. 



GREEN MINERAL PIGMENTS. 243 

Brunswick Green, Green Verditer, is a mixture of copper 
arsenite, copper hydroxide and gypsum. It is prepared by 
dissolving 100 parts of copper sulphate in water, adding 
a solution of J part of arsenious acid and 10 parts of 
anhydrous potassium carbonate, and finally precipitating 
with milk of lime made from 20 parts of quicklime. The pale 
to bluish green precipitate is washed and pressed into flat 
cakes, which are brought into the market and used as lime 
colours. This pigment is not adapted for use in oil ; when 
applied in that medium it darkens considerably. 

Neuwied Green is made in almost exactly the same 
manner as Brunswick green, the chief difference being that 
a larger quantity of arsenic is used, and thus a product 
obtained inclining more to green than to blue. As a rule 
2 to 2| parts of arsenious acid are used to 100 parts of 
copper sulphate. The colours found in commerce under 
this name very often consist of mixtures of emerald green 
and finely ground barytes. 

Copper Oxychloride was at one time largely used. At 
present it is scarcely employed ; it has been replaced by 
handsomer and cheaper pigments. Copper oxychloride is 
made by exposing a wet mixture of copper, copper sulphate 
and common salt to the action of the air. To 111 parts 
of copper sulphate 112'5 parts of copper and the same 
quantity of common salt are employed. The mixture is 
made into heaps, which are wetted and from time to time 
shovelled about, so that the air may come in contact with 
all portions of the mass, and the copper chloride be converted 
into basic chloride. It is advisable to allow the heaps to 
dry frequently after they have been turned over, so that 
the air may more readily penetrate into the interior. Copper 
oxychloride has a pale green colour of little brilliance. On 
this accoiv.it it is hardly ever used now ; it forms the raw 
material f jr the preparation of Bremen green. 



CHAPTEK XXIX. 

EMERALD GREEN. 

THERE are many versions as to the discovery of this pigment. 
According to one it was first made in Schweinfurt by Euss 
and Sattler ; another version gives it that Mitis of Vienna first 
made it on the large scale. It is possible, as has been the 
case with many chemical products, that this pigment was 
simultaneously discovered by both. The method of making 
the pigment was at first a secret possessed by few. Not 
until the publication of a method by Liebig in 1822 was 
the preparation of emerald green generally known. In recent 
times the industry has supplied other colours which are less 
poisonous. The time should not be far distant in which this 
handsome but highly poisonous pigment shall no longer be 
found in commerce. 

The circumstance that the poisonous nature of the pig- 
ment and cautions as to its use were given great publicity, 
brought about the discovery of the countless names under 
which it has been sold. It was endeavoured by re-naming the 
pigment to pass it off as a different substance. All the pig- 
ments which have been sold under the names of Mitis, moss, 
patent, new, king's green, etc., are either pure emerald green 
or mixtures of it with barytes or gypsum, wlich additions 
were intended to alter the shade of the green t.ud make it 
appear to the buyer as a new pigment. 

In chemical composition emerald green is a compound 



EMEEALD GREEN. 245 

of copper arsenite and copper acetate. It has the following 
formula : 

3 (GuO . AajOg) + Cu (C 2 H 3 O 2 ) 2 . 

This compound rarely comes into commerce in the pure 
state ; it is generall} 7 mixed with chrome yellow or lead 
sulphate, which are added with the object of producing 
shades between yellowish green and dark green. Emerald 
green is a compound of constant composition. When made 
by different processes it has the same properties. Under the 
microscope it is seen to consist of crystals ; when the pure 
substance is ground it becomes paler, the intensity of the 
colour being diminished by the breakage of the crystals. The 
colour of emerald green is not altered by artificial light. This 
is a most valuable property ; the majority of green pigments 
appear yellowish-green in artificial light. 

Emerald green can be made on the large scale by two 
processes ; these differ according to the copper salt used, 
which is either the acetate (verdigris) or the sulphate. The 
latter is by far cheaper than verdigris, which is now seldom 
used for this purpose. 

Manufacture of Emerald Green from Verdigris, By boil- 
ing for several hours, 70 parts of ordinary verdigris are 
dissolved in water ; in a second vessel 100 parts of arsenious 
acid are dissolved in 1,500 parts of hot water ; the verdigris 
solution is filtered through a cloth into a larger wooden 
vessel and mixed with two-thirds of the arsenic solution. The 
mixture is well stirred and allowed to stand for some hours. 
A precipitate of dull green colour is formed. This consists 
of copper arsenite. After three to four hours the remaining 
one-third of the arsenic solution is added ; the precipitate 
then changes into the double salt of which emerald green 
consists. 

The use of such dilute solutions as are given in these 



246 MINERAL AND LAKE PIGMENTS. 

directions is essential when a product of particularly good 
colour is required. Emerald green is a crystalline substance 
which is deeper in colour in proportion to the size of its 
crystals. Large crystals can only be formed from very dilute 
solutions ; when strong solutions are used, the emerald green 
has never the deep colour possessed by the product of dilute 
solutions. In the place of ordinary (basic verdigris) the so- 
called distilled verdigris (neutral copper acetate) can be used, 
of which seventy-six parts are required for the quantity of 
arsenious acid given. The emerald green made from "dis- 
tilled" verdigris is of a yet deeper colour than that made 
from ordinary verdigris, but is considerably dearer. Such 
emerald green is commercially described, very incorrectly, as 
distilled emerald green. 

The method given by Liebig is as follows : 4 parts of basic 
verdigris and 3 parts of arsenious acid are separately dis- 
solved in boiling vinegar ; the liquids are mixed and evapo- 
rated until a precipitate of emerald green separates. 

According to Ehrmann and Kastner, 70 parts of verdigris 
and 56 to 63 parts of arsenious acid are separately dissolved, 
the boiling solutions mixed and boiled until the dirty green 
precipitate changes into a bright green. In consequence of 
the rapid formation of the precipitate due to the boiling, the 
colour is pale green ; if a dark-coloured product is required, 
the solutions are allowed to cool before mixing. After 
several days a very dark-coloured precipitate separates from 
the liquid ; it consists of comparatively very large crystals of 
emerald green. 

Manufacture of Emerald Green from Copper Sulphate, 
There are two processes. Either the solution of copper 
sulphate is mixed with an acetate, by which it is converted 
into copper acetate, and then with arsenious acid, or copper 
arsenite is precipitated from the solution of copper sulphate 
and then converted into emerald green by treatment with 



EMERALD GREEN. 247 

acetic acid. If acetic acid can be cheaply obtained, the latter 
process is to be recommended. 

Braconnet gives the following directions : 3 parts of copper 
sulphate are dissolved in a little boiling water, the hot solu- 
tion is mixed with a hot solution of 4 parts of arsenious acid 
and four parts of potassium carbonate ; the precipitate is then 
treated with 3 parts of pyroligneous acid. The dull-coloured 
precipitate at first formed is changed by the acetic acid to a 
bright green. The colour is more quickly produced when 
the liquid is heated nearly to boiling for several hours. 
When the desired shade appears the boiling is stopped and 
the precipitate at once separated from the liquid, by long 
contact with which it would give up arsenious acid and 
thus lose in shade. 

According to Fuchs, 5 kilogrammes of lime and 25 kilo- 
grammes of copper sulphate are dissolved in acetic acid, then 
a boiling solution of 25 kilogrammes of arsenious acid is 
added. The precipitate is at once formed ; it is immediately 
separated from the liquid, washed, and dried ; the top liquor, 
with the addition of arsenious acid, can be again used to 
precipitate the copper lime solution. 

Emerald green has an extremely bright colour which 
surpasses other green mineral pigments in beauty. The 
larger are the microscopic crystals of which it consists the 
deeper is the colour. In order to obtain the desired deep 
shade of emerald green it is necessary to use very dilute 
solutions ; from strong solutions the precipitate is instan- 
taneously produced, in which case it is impossible to obtain 
large crystals. The covering power of emerald green, in 
consequence of its crystalline nature, is less the deeper 
the shade. It cannot be regarded as a permanent colour : 
very dilute alkalis and acids attack it. By the action of 
sulphuretted hydrogen it is discoloured, owing to the forma- 
tion of black copper sulphide. It ought not to be used upon 



248 MINERAL AND LAKE PIGMENTS. 

lime, which withdraws acetic acid and produces yellowish- 
green copper arsenite. 

This pigment should not be used for distempering or for 
colouring wall papers. By the action of the atmosphere on 
the arsenic it contains, arseniuretted hydrogen is formed an 
extremely poisonous gas, very small quantities of which are 
sufficient to cause serious illness in the case of susceptible 
people. In spite of its poisonous nature, emerald green is 
still largely used. All possible shades of green can be pro- 
duced from it, yellowish green or pale green, by admixtures 
of yellow pigments, such as chrome yellow, or white pig- 
ments, such as barytes or white lead. Many qualities of 
green known as palette green, Basle green, etc., are generally 
composed of such mixtures. 

Mitis Green or Vienna Green is obtained by dissolving 
verdigris in acetic acid and adding a boiling solution of white 
arsenic in water ; acetic acid is then added until the pre- 
cipitate dissolves. On boiling the clear solution a green 
precipitate forms, which, when dry, is a deep bluish-green 
of characteristic shade. This pigment is now practically 
disused because of its poisonous nature, which is equal to 
that of emerald green. 

Copper Stannate, This green pigment, also known as 
Gentele's green, can be made by two methods. Gentele 
gave the following process : 59 parts of tin are converted 
into stannic chloride by solution in aqua regia ; a solution of 
125 parts of copper sulphate is added and copper stannate 
is precipitated from the mixture by caustic soda, on washing 
and igniting it acquires a pretty green colour. It may also 
be made by fusing 59 parts of tin with saltpetre ; the potas- 
sium stannate is then dissolved in dilute caustic soda and 
copper sulphate solution added ; the precipitate is washed and 
ignited. 

Copper stannate is tolerably durable, in this respect it 



EMERALD GREEN. 249 

considerably surpasses emerald green. Only sulphuretted 
hydrogen has any considerable action upon it, turning it 
to a dirty brownish -green hue. 

Kuhlmann's Green is a basic chloride of copper, obtained 
by heating 2 equivalents of lime with a solution of 3 equi- 
valents of copper chloride. It is important that the copper 
salt should be present in excess. In shade Kuhlmann's 
green is very similar to emerald green, with which it agrees 
in retaining its colour by artificial light. It is a somewhat 
less pure green than emerald green ; the difference is only 
perceived when the two pigments are directly compared. 
When, however, it is considered that it is far more permanent 
than emerald green and cheaper, a more extended use is 
indicated than is yet the case. 

Eisner's Green is a species of lake, made by mixing a 
solution of copper sulphate with a decoction of fustic, adding 
a small quantity of stannous chloride and precipitating with 
caustic soda. To 100 parts of copper sulphate 10 to 14 parts 
of stannous chloride are used. According as the copper salt 
or fustic extract predominates, the colour of the precipitate 
inclines to blue or yellow. 

Eisner's green is also sold under the name of " non- 
poisonous green". This description is incorrect; the pig- 
ment is indeed free from arsenic, but is poisonous on account 
of the copper it contains. 

Casselmann's Green approaches emerald green in bright- 
ness. It consists of a compound of copper sulphate with 
copper hydroxide and water, CuS0 4 .3Cu(OH) 2 .JH 2 0. This 
pigment is obtained by mixing solutions of 4 equivalents of 
copper sulphate and 3 equivalents of sodium acetate at a 
certain temperature. The author's experiments have shown 
that the best results are obtained when the solutions are 
mixed at a temperature of about 10U C. With this object 
the solutions are placed in vessels standing in a pan of 



250 MINEBAL AND LAKE PIGMENTS. 

boiling water ; when their temperatures have risen to about 
100 C. they are quickly mixed, the mixture stirred, and 
the precipitate allowed to settle. When the precipitate is 
cautiously treated with very weak caustic soda solution a 
rather deeper colour is obtained. This treatment should not 
be continued too long, or the precipitate may acquire an ugly 
bluish shade. 

Lime Green is a mixture of copper arsenite and calcium 
sulphate. It is thus a pigment which ought no longer to 
be used, on account of its poisonous properties. It is made 
by boiling milk of lime with excess of arsenious acid so 
long as the latter is dissolved. To this solution of calcium 
arsenite, copper sulphate solution, is added so long as a 
precipitate is formed. A mixture of copper arsenite and 
calcium sulphate is precipitated. 

Patent Green is similar in composition to lime green. 
A solution of calcium acetate is made by adding quicklime 
or pure powdered limestone to acetic acid ; copper sulphate 
solution is then added, when gypsum is precipitated and 
copper acetate remains in solution. A hot solution of 
arsenious acid is next added, and the precipitated copper 
salt mixed with the gypsum at the bottom of the vessel. 

Copper Borate. A pale green precipitate of copper borate 
is obtained by adding a solution of 5 parts of borax to a solu- 
tion of 2 parts of copper sulphate. It must be dried at a 
very moderate temperature, or it will decompose. When 
perfectly dry the precipitate can be heated to a red heat 
without decomposition ; according to the temperature em- 
ployed different shades are obtained. It is most convenient 
to take tests out of the crucible from time to time during the 
ignition, and to quickly cool the crucible when the desired 
shade is obtained. When levigated this pigment may be 
used in oil, or as a porcelain colour. 

Copper Silicate (Egyptian Blue). When a solution of 



EMERALD GREEN. 251 

water-glass is added to a solution of copper sulphate a pale 
green precipitate of copper silicate is obtained, which can 
be heated to redness without alteration. When 70 parts 
of white quartz sand, 15 parts of copper oxide, 25 parts of 
chalk and 6 parts of soda are fused together a glass is 
produced which, after pouring into water, grinding and 
levigating, exhibits a pretty blue and very permanent colour. 
It appears from the examination of the colours of the 
Egyptian mural paintings that this pigment was already 
known to the ancient Egyptians. 



CHAPTEE XXX. 

VERDIGRIS. 

VEEDIGEIS is little used as a pigment, but is important to 
the colour maker, because copper colours are made from it. 
In wine-growing countries it is made from the marc at small 
expense and without much labour. 

In commerce several kinds of verdigris are distinguished ; 
they differ in physical and chemical properties. All verdigris 
consists of copper acetate, either alone or combined with 
varying quantities of copper hydroxide. 

Blue Verdigris is made in large quantities in France and 
is commonly known as French verdigris (vert de gris naturel}. 
It has the following formula : 

Cu (OH) (C 2 H 3 2 ) . 2H 2 0. 

The process adopted in southern France, especially in the 
neighbourhood of Montpellier, is as follows : copper plates are 
placed in layers in heaps of the freshly pressed grape residues, 
which always contain a certain quantity of must even when 
the most powerful presses are used. The process is conducted 
either in large heaps or in pots ; in the first case care roust be 
taken that the heaps are in a room whose temperature does not 
lie below 15 C. It is important to maintain this temperature, 
because the formation of acetic acid takes place with suffi- 
cient rapidity only at temperatures not below 12 to 15 C. 
Too high a rise of temperature in the heaps must also be 
avoided or a considerable quantity of acetic acid will be 
volatilised. The heaps should therefore not be made too large, 



VERDIGRIS. 253 

otherwise the temperature cannot be kept within the proper 
limits. 

The residues contain a considerable quantity of sugar, 
which can be transformed by fermentation into alcohol. If 
air has free access, the alcohol is at once oxidised to acetic 
acid, which is recognised by the acid smell given off. In 
order to make possible the entry of air into the interior of 
the heaps, rectangular wooden bars are introduced in piling 
up the heaps. These are carefully withdrawn when the heap 
is finished, so that it is traversed by passages through which 
air can enter. By the simultaneous action of air and acetic 
acid copper acetate is formed, and since copper is always in 
excess, the acetate produced is a basic salt. In consequence 
of the chemical processes, the temperature of the heap rises 
to 35 to 40 C. ; it is endeavoured to attain this temperature 
by artificial heat. It has already been stated that too great 
a rise, of temperature is accompanied by a considerable 
evaporation of acetic acid. The process is then finished in a 
short time, but the loss of acetic acid is remarkable. When 
the temperature is so regulated that it varies between 25 
and 30 C. the process takes a normal course four to five 
days may be regarded as its duration. 

The grape residues are effective on ace )unt of the sugar, 
which is transformed into alcohol and then into acetic acid. 
Eesidues which have been extracted with water after pressing 
should not be used in the manufacture of verdigris ; they 
contain but little sugar and hence undergo a very feeble 
acetic fermentation. When such residues are used the 
verdigris is often accompanied by black spots of copper sul- 
phide upon the plates ; this is due to incipient decomposition 
of the residues. 

As a practical test for the termination of the process a 
strip of copper is plunged into the heap and left for some 
hours. It should be covered by a uniform coating of 



254 MINEEAL AND LAKE PIGMENTS. 

verdigris ; if it is covered with small drops that is a sign that 
the process is not completed and the heaps must be left 
some time longer. The course of the process can be simply 
and safely followed by means of a thermometer. This is 
placed in a perforated copper tube and plunged into the 
interior of the heap. A continuous rise of temperature in- 
dicates a steady increase of chemical action ; the temperature 
of the room is regulated in accordance. When the tempera- 
ture in the interior of the mass begins to fall the process is 
nearing the end. More heat is now supplied to assist the 
chemical action. The process is at an end when, in spite 
of external heat, the temperature of the heap decreases and 
approaches the temperature of the air of the room. 

Verdigris can also be made by placing sheet copper and 
grape residues in layers in pots, which are deposited in a 
room of fairly uniform temperature, such as a cellar. This 
method has the advantage that the formation of the ver- 
digris is finished in a shorter time, which is more than 
counterbalanced by the labour and expense of filling the 
pots. 

The copper plates, when removed from the heaps or pots, 
are covered by a crust of thin needle-shaped crystals of a 
bright green colour. They are shaken to free them from the 
adhering grape skins and seeds, and then treated with weak 
vinegar, in which they are dipped ; they are then left 
standing on edge for several days. This treatment with 
vinegar has the object of converting the neutral salt formed 
on the plates into basic salt. The dipping into vinegar and 
exposure to air are repeated six to eight times, when the 
originally pure green crust on the plates is gradually changed 
into bluish-green verdigris. These processes are continued 
until the plates are covered uniformly by a layer of verdigris 
about 5 centimetres thick. The crust is then scraped off by 
copper knives and stirred with water to a paste, which is 



YERDIGKRIS. 255 

pressed in leather bags in rectangular moulds. The lumps 
are" then slowly dried in the air. 

Plates which have been once used give a larger quantity 
of verdigris in a second operation, which is to be ascribed to 
the fact that they possess a larger surface than unused 
plates. New plates are made more susceptible to the further 
action of the acid by dipping in strong vinegar, by which 
they become covered with a layer of copper acetate. 

The verdigris obtained by this method consists of crystal- 
line scales of a pale bluish-green colour, which produce a pale 
blue powder. In crude verdigris grape seeds are often found, 
and occasionally stalks and pieces of metallic copper. These 
admixtures are a consequence of the method of manufacture 
and cannot be regarded as adulterants. Gypsum is to be 
regarded as an adulterant. Verdigris often contains basic 
copper carbonate. 

Verdigris behaves in a peculiar manner with water. In 
contact with a small quantity it swells to a bluish-green 
mass, which becomes quite warm. Neutral copper acetate 
and the basic salts 2Cu(C 2 H 3 2 ) 2 . Cu(OH), . 5H 2 and 
Cu(C 2 H 3 2 ) 2 .2CuO.UH,0 are formed, the latter of which 
is insoluble. 

By the addition of a larger quantity of water the basic com- 
pounds are decomposed, neutral copper acetate dissolves, and 
a mixture remains of the compound Cu(C 9 H 3 2 ) 2 . 2CuO . liH 2 
and a brown basic acetate containing still less acetic acid. 
On account of this peculiar behaviour of verdigris towards 
water some care is required when it is used as a water 
colour. A very dilute solution is not green, but has an 
indefinite shade. 

Distilled or Crystallised Verdigris consists of neutral 
copper acetate, Cu(C 2 H 3 2 ) 2 .H 2 0. It can be made from 
blue verdigris by treating with the amount of acetic acid 
required to completely neutralise the copper oxide, or by 



256 MINEEAL AND LAKE PIGMENTS. 

decomposing copper sulphate with the acetate of a metal 
which forms an insoluble or difficultly soluble sulphate. 

Crystallised verdigris is very simply made from ordinary 
verdigris ; the latter, whilst still moist, but containing only a 
small quantity of water, or the acetic acid would be too 
largely diluted, is brought into a pan and strong acetic acid 
poured over. Strong pyroligneous acid may be used, its 
empyreumatic odour is without influence on the quality of 
the product. The pan is heated until the contents almost, 
but not quite, boil. They are frequently stirred so that the 
particles at the bottom are brought in contact with the acid. 
A dark green solution is formed ; when its colour no longer 
increases in depth it is allowed to stand until suspended 
solids sink to the bottom. The clear solution is then drawn 
off and rapidly evaporated in shallow copper pans. It is 
important that the liquid should not contain an appreciable 
quantity of free acid. When a crystalline skin begins to 
form on the surface of the solution it is drawn off into the 
crystallising vessels. These are made of glazed earthenware ; 
wooden rods are placed in them, on which the crystallisation 
takes place. In order that large crystals shall be obtained, 
it is necessary to maintain a regular temperature in the room 
in which the crystallising vessels are placed. It is heated at 
the commencement, and the temperature is allowed to sink 
a little towards the end of the operation. From twelve to 
fourteen days are required to obtain large crystals. The mass 
of verdigris crystals adhering to each rod weighs 2'5 to 3 
kilogrammes. The residual liquid is a saturated solution of 
verdigris, and is used in the next operation. It is only 
necessary to heat it with the residue from the first operation 
and some acetic acid in order to obtain a crystallisable solu- 
tion of verdigris. 

The residue in the pan contains metallic copper, grape 
stems and seeds, and basic copper acetate. The copper is 



VEBDIGRIS. 257 

extracted by moistening with acetic acid and exposing to 
the air, when verdigris is formed, which' is added to a later 
batch. 

It is often advisable to make verdigris from a soluble 
copper salt ; the process varies somewhat according to the 
salt employed. From copper sulphate verdigris is obtained 
by the action of calcium acetate. When solutions of these 
compounds are mixed, calcium sulphate separates as a white 
precipitate, whilst a solution of the easily soluble copper 
acetate is left. It is only necessary to mix the solutions 
in equivalent proportions, separate the liquid from the pre- 
cipitated gypsum and evaporate. Gypsum is somewhat 
soluble in water, so that when the solution is evaporated 
a double salt separates out first. This consists of copper 
calcium acetate ; it may be used as a pigment, but is of less 
value than verdigris. It is advisable to prevent the formation 
of this double salt, which is accomplished by adding copper 
sulphate to a slightly acid solution of calcium acetate until 
a precipitate is no longer produced. The solution of copper 
acetate is boiled for several hours to effect the separation of 
the dissolved gypsum ; at the same time the iron contained 
as impurity in the copper sulphate is also precipitated. The 
purified solution of verdigris is concentrated and allowed to 
cool, when a further quantity of gypsum separates ; the solu- 
tion is then evaporated to crystallisation. 

If barium salts and lead acetate are cheap, barium acetate 
can be obtained by double decomposition. When copper 
sulphate is added to the solution of barium acetate so long 
as a precipitate is formed, a solution of verdigris is produced, 
which only requires evaporation to crystallise. The pre- 
cipitate is barium sulphate, and can be used as enamel white. 
It obstinately retains a quantity of copper, which, though 
small, is sufficient to give it a greenish tint. By several 
washings with acetic acid this copper is removed ; the acid 

17 



258 MINEKAL AND LAKE PIGMENTS. 

can be used to obtain fresh quantities of verdigris, so that 
none is lost, and the copper retained in the precipitate is 
regained. The precipitate, after treatment with acetic acid, 
only requires thorough washing with water to produce per- 
manent white, satisfactory in every respect. 

Ammonia may also be used in the manufacture of ver- 
digris from copper sulphate. Strong ammonia is left in 
contact with copper sulphate in a covered vessel for several 
hours ; gentle warming accelerates the formation of copper 
ammonium sulphate. The liquid has then a fine blue colour; 
it should not contain sufficient ammonia to be perceptible 
by the smell. It is then gently warmed and acetic acid 
gradually added. On continued heating small green crystals 
separate from the solution, which is boiled with continual 
additions of acetic acid in small quantities until small crystals 
appear on the surface, when it is allowed to cool and the 
crystals are strained off. 

"Distilled" verdigris forms deep bluish green crystals, 
which effloresce slightly in the air ; they dissolve in 5 parts 
of boiling water and in 13 '4 parts at 20 C. When the 
solution is boiled for some time the tribasic acetate separates ; 
the liquid becomes brown and acquires an acid reaction. 

German Verdigris does not differ essentially from ordinary 
verdigris. It is a compound of the following basic acetates : 
2Cu(C 2 H 3 2 ) 2 .CuO and Cu(C,H 3 O 2 ). 2 .2CuO. The method is 
similar to that in which grape residues are used ; where 
acetic acid is cheap the process is especially appropriate. 
In Sweden, where pyroligneous acid is made in large quan- 
tities and copper is also cheap, verdigris is made by arranging 
copper plates and flannel in alternate layers. The plates are 
frequently turned over and the flannel kept saturated with 
acetic acid. The same chemical reactions occur as in the 
process in which grape residues are used. When a layer of 
verdigris has formed on the copper plates they are taken 



VEKDIGKIS. 259 

apart, exposed to the air, and frequently wetted. The for- 
mation of the verdigris is thus completed. The difference 
between this process and that adopted in wine-producing 
countries lies simply in the use of pure acetic acid ; the 
product is free from the mechanical impurities introduced 
by the grape residues. In properties German or Swedish 
verdigris is completely identical with blue verdigris. 



CHAPTEB XXXI. 

CHROMIUM OXIDE. 

Chrome Green. Although very different substances are 
commercially known as chrome green, this name is usually 
applied to the very valuable pigment which consists of pure 
chromium oxide. There is hardly another pigment for the 
preparation of which such varied directions have been given 
and whose shade varies so much according to the manner 
of production. 

The cheapest process is to ignite potassium bichromate 
with sulphur, extract the mass with very dilute sulphuric 
aci_djmd washj^jcesidue^ The chromic acid is reduced by 
the sulphur ; when the mass is treated with sulphuric acid 
sulphur dioxide is evolved, potassium sulphide and sulphate 
are dissolved, whilst pure chromium oxide remains. The 
larger the quantity of sulphur used the paler is the chromium 
oxide. The purity of the potassium bichromate has con- 
siderable influence on the shade of the pigment ; if iron is 
present in any quantity a discoloured product always results. 
Convenient proportions are : 19 parts of potassium bichromate 
and 4 parts of sulphur, which produce 9'33 parts of chromium 
oxide. If potassium bichromate free from iron cannot be 
obtained, the shade of the product is somewhat improved by 
treatment with dilute hydrochloric acid, in which ferric 
oxide dissolves more easily than chromium oxide. When the 
latter has been strongly ignited it is soluble with very great 
difficulty. 



CHEOMIUM OXIDE. 261 

There are many formulae for the preparation of chromium 
oxide by means of sulphur : in all the above statement holds 
good, that the more sulphur the paler the product. 

According to A. Casali, a chrome green which satisfies 
every requirement is obtained by strongly igniting 1 part of 
potassium bichromate with 3 parts of burnt gypsum. After 
ignition the mass is boiled with very dilute hydrochloric acid. 
In this process the chromic oxide is produced according to 
the following equation : - 

2K 2 C 2 7 + CaS0 4 = 2Cr 2 O 3 + 2K 2 SO 4 + 2CaO + 30 2 . 

On boiling with hydrochloric acid the lime is dissolved ; when 
the liquid remains distinctly acid after long boiling it is 
poured off and the chromium oxide washed with hot water 
and dried. 

In another method ammonium chromate is very gradually 
heated. At a certain temperature the salt suddenly becomes 
incandescent and is converted into a dark green, almost black, 
mass which has a very similar appearance to dry tea-leaves. 
The green obtained after washing and powdering is hand- 
somer the lower the temperature of the decomposition. 

Chromium oxide may also be made in the wet way, but 
the product does not compare in beauty with that obtained 
in the dry way. When soda solution is added to a solution 
of chrome alum, a greyish green precipitate of chromium 
hydroxide is formed ; on ignition of the washed precipitate 
pure chromium oxide remains. 

In a similar manner chromium oxide is produced by pre- 
cipitating with soda solution the solution of chromium chloride 
obtained by adding hydrochloric acid to a solution of potassium 
bichromate, and then alcohol in small quantities, so long as a 
reaction occurs and the green liquid becomes deeper in colour. 

A handsome and bright chrome green is only obtained 
when the potassium bichromate is quite free from iron, a 



262 MINERAL AND LAKE PIGMENTS. 

very small quantity of which has a very harmful effect upon 
the brightness of the pigment. The author has found that 
there is no great difficulty in rendering commercial potassium 
bichromate tolerably free from iron by recrystallisation. As 
much of the salt is dissolved in boiling water as it will take 
up, the boiling solution is quickly filtered and rapidly cooled 
with continual stirring ; the fine crystalline meal is left on a 
strainer until no more liquid filters through, and then washed 
with a little cold water to remove the mother liquor. The 
salt obtained by this simple operation is of great purity and 
always produces chrome green of a good shade. 

When chrome green contains ferric oxide it shows a 
blackish colour. From such chrome green, which has little 
value, a product of more pure colour can be obtained with a 
little care. By treatment with dilute hydrochloric acid the 
ferric oxide dissolves, even when it has been somewhat 
strongly ignited ; chromium oxide loses almost all solubility 
in dilute hydrochloric acid after heating at a relatively low 
temperature. To facilitate the solution of the ferric oxide, 
the green to be treated is finely powdered ; it is then covered 
with a mixture of equal parts of hydrochloric acid and water. 
After several days the mass is strained and washed with 
pare water until the acid reaction disappears. Comparative 
analyses have shown that almost the whole of the ferric oxide 
can be removed from chrome green in this manner : the bright- 
ness of the colour is considerably increased. It is, however, 
advisable to employ materials free from iron in the manufac- 
ture of chrome green. 

Chromium oxide can be obtained as a colour of particular 
brightness, according to the process of Leune, by precipitating 
it very slowly from a solution in which it is contained in the 
green modification. For this purpose a solution of chrome 
alum is boiled until the violet colour has changed to green ; 
the green modification of chromium oxide is now contained in 



CHROMIUM OXIDE. 263 

the solution. It is cooled to about 10 C., and either freshly 
precipitated alumina or zinc carbonate added, very gradually 
and in small quantities. A fine green shade of chromium 
oxide separates ; after washing and drying it is obtained in a 
condition in which it can be used as a pigment. 

Chromium oxide can indeed be obtained by this process, 
but it is not distinguished by particular beauty ; the author 
has never been able to obtain, even by using the precipitant 
in very small quantities, a chromium oxide surpassing in 
shade that obtained by the direct precipitation of a chromium 
salt with an alkali. 

The emerald green described in the following chapter is 
obtained in a similar manner by precipitation with zinc 
oxide. 



CHAPTEE XXXII. 

OTHER GREEN CHROMIUM PIGMENTS. 

Guignet's Green is a chromium oxide made in the dry way ; 
it is obtained by grinding 1 part of potassium bichromate 
with 3 parts of pure boric acid and water, drying and 
heating to a dark red heat with access of air. The hot mass 
is brought into water ; by boiling repeatedly with water it is 
freed from the boric acid which it obstinately retains. In 
order to remove the last portions of boric acid it is necessary 
to boil with sulphuric acid and then with caustic soda ; this 
treatment may be omitted for technical purposes, since the 
small quantities of boric acid contained in the pigment are 
not harmful. 

Guignet's green is distinguished by the great resistance it 
offers to chemical reagents ; it is largely used in painting and 
calico printing. Pale shades are obtained by additions of 
barytes. 

Emerald Green, This pigment must not be confounded 
with the more important copper compound previously de- 
scribed, to which the name emerald green is usually applied. 
It consists of chromium hydroxide obtained by precipitating 
a solution of the green modification of a chromium salt by 
zinc hydroxide. After careful washing it has a dull green 
shade ; it is a very durable pigment. 

Chrome Green Lake is a mixture of chromium oxide with 
alumina ; it is obtained by precipitating a solution of alum 
and a chromium salt by a soda solution. The precipitate, 



OTHER GREEN CHROMIUM PIGMENTS. 265 

which contains aluminium and chromium hydroxides, on 
ignition takes a paler shade in proportion to the quantity 
of alumina it contains. For the chromium salt a solution 
of potassium bichromate, which has been allowed to stand 
with sulphuric acid and alcohol until if has acquired a pure 
green colour, may be used. 

Turkish Green. This pigment, which retains its fine green 
colour in artificial light, is prepared in a peculiar manner : 
40 parts of alumina free from iron, 30 parts of cobalt car- 
bonate and 20 parts of chromium oxide are thoroughly ground 
in a mortar. The mixture is placed in a porcelain tube, which 
is exposed to a strong white heat whilst pure oxygen is led 
through. The oxygen may be replaced by air if it is previously 
heated and under pressure. 

Another recipe for Turkish green is to intimately mix 4 
parts of freshly precipitated alumina with 3 parts of cobalt 
carbonate and 2 parts of chromium oxide, heat the mixture 
in a crucible at a white heat, powder and levigate. Turkish 
green has a characteristic bluish-green colour ; the shade is 
inclined to blue or green by increasing the quantity of cobalt 
carbonate or chromium oxide. 

Leaf Green is a pale green, very durable pigment similar to 
chrome green lake. It is obtained by igniting mixtures of 
chromium oxide with pure aluminium hydrate ; the paleness 
of the shade is in proportion to the amount of alumina. 

Chromium Phosphate Pigments, Several chromium phos- 
phates are used as pigments ; the more important are described 
here. 

Arnaudan's Green is chromium metaphosphate ; it is ob- 
tained by intimately mixing 128 parts of neutral ammonium 
phosphate with 149 parts of potassium bichromate by long 
grinding, and carefully heating the mixture at 170 to 180 C., 
but not higher, until the mass is pure green. It is then 
brought into hot water and thoroughly washed ; it has a 



266 MINEEAL AND LAKE PIGMENTS. 

very handsome green colour, which is not altered in artificial 
light. 

Ammonium arsenate may be used in the place of ammo- 
nium phosphate ; a handsomer green is then obtained, which 
is, however, very poisonous. 

Plessy's Green is essentially a chromium phosphate mixed 
with variable amounts of chromium oxide and calcium phos- 
phate. It is obtained by boiling a solution of 1 part of 
potassium bichromate in 10 parts of water with 3 parts of a 
solution of acid calcium phosphate and 1 part of sugar, 
until the whole has become deep green. The chromic acid 
is reduced by the sugar, so that the precipitate contains 
chromium phosphate, chromium oxide and neutral calcium 
phosphate. 

These two chromium phosphate greens are very stable 
towards chemical reagents, and very durable towards atmo- 
spheric influences. 

Schnitzer's Green. Thirty-six parts of sodium phosphate 
are melted in its water of crystallisation, 15 parts of potas- 
sium bichromate and 14 parts of Rochelle salt are then added. 
A large dish must be used for the fusion, since the mass 
effervesces. The colour changes gradually from yellow to 
green. When it is pure green the heating is stopped, and, 
after cooling somewhat, as much hydrochloric acid is added 
as the mass will absorb ; then, after standing some time, it is 
washed with cold and finally with boiling water. On account 
of their great durability, the chromium phosphate pigments 
are well adapted for paperhangings, calico printing and oil 
painting. 

Chromaventurine is a glass coloured green by chromium 
oxide. It is hardly used as a painters' colour in the 
ordinary sense of the term ; it is applied in porcelain paint- 
ing as an under-glaze colour, and is largely used for colouring 
glass. 



OTHER GREEN CHROMIUM PIGMENTS. 267 

Chromaventurine is most simply made by the process of 
Pelouze : 250 parts of quartz sand, 100 parts of soda, 50 
parts of calcium carbonate and 40 parts of potassium 
bichromate are fused together. The presence of iron would 
have a very harmful effect on the shade of the product. 
Quartz sand generally contains a small quantity of ferric 
oxide, which should be extracted by treatment with strong 
hydrochloric acid when a product of the best colour is 
required. The calcium carbonate should also, as far as 
possible, be free from ferric oxide. 

Chrome Blue (Gamier). A mixture of 48'62 parts of 
potassium chromate with 65 parts of fluorspar and 157 parts 
of silica is fused in a crucible lined with coal-dust. 



CHAPTEK XXXIII. 

GKEEN COBALT PIGMENTS. 

FEOM mixtures of chromium oxide, cobalt carbonate and 
alumina in different proportions a series of pigments can 
be obtained varying in shade between pale blue and bluish 
green. On account of the great stability of these colours 
at the highest temperatures they are of great importance 
in porcelain painting, for which they are principally used. 
It is necessary that the porcelain should be free from iron, 
since ferric oxide forms a black compound with chromium 
oxide, very small quantities of w r hich would be sufficient 
to considerably damage the fineness of the colour. 

Cobalt Green, which is also known as Einmann's green 
or zinc green, is a compound of the oxides of cobalt and 
zinc. It is always produced when a cobalt compound is 
ignited with zinc oxide. Kinmann's green has not so deep a 
colour as the poisonous emerald green, but it is distinguished 
by extraordinary durability, and deserves to be more largely 
used than it is at present. This pigment is most simply 
made by precipitating the mixed solutions of a zinc salt 
and a cobalt salt. It is also formed by moistening pure 
zinc oxide with a cobalt solution and igniting. 

In precipitating the mixed solutions of zinc and cobalt 
salts products of different shades are obtained, according to 
the proportions in which the salts are used. If equivalent 
quantities are used, an almost black product is obtained, 
quite useless as an artists' colour. The best result is obtained 



GEEEN COBALT PIGMENTS. 269 

by mixing intimately pure precipitated cobalt carbonate with 
zinc oxide and igniting the mixture ; a mixture of 9 to .10 
parts of zinc oxide with 1 to 1/5 part of cobalt carbonate 
gives colours between pale green and dark green. 

Especially fine Kinmann's green is obtained by igniting 
cobalt arsenate with zinc oxide and arsenious acid. The 
addition of the last-named substance can only have the 
object of preventing the temperature from rising too high, 
which might injure the beauty of the colour ; arsenious 
acid is volatile at a fairly low temperature. 

Cobalt green is also obtained by evaporating a solution of 
cobalt nitrate and zinc nitrate and igniting the residue, or by 
igniting a mixture of the sulphates. In the latter case a 
tolerably high temperature is necessary in order that the 
sulphates may be decomposed. 

The author has found that a particularly handsome 
product is obtained by mixing pure zinc oxide with a dilute 
solution of cobalt chloride, drying and then slowly heating 
to redness in a crucible with a well-fitting lid. Towards the 
end of the operation the heat is considerably increased and 
maintained for a short time, after which the mass is rapidly 
cooled. 



CHAPTEK XXXIV. 

GREEN MANGANESE PIGMENTS. 

Manganese Green, Rosenstiehl's Green, This pigment, 
which is handsome but difficult to produce, consists of 
barium manganate. It can be obtained by several methods, 
all of which produce a good green, but with very unequal 
properties. Manganese green made from barium nitrate 
has little durability ; when made from barium hydrate it is 
more costly, but very durable. 

Barium manganate is most easily obtained by precipi- 
tating a boiling solution of potassium manganate by barium 
chloride. The almost blue precipitate becomes nearly white 
when washed and dried, but wher it is gradually heated on 
a porcelain plate to a dark red heat it acquires a fine green 
colour. The heating must be carefully conducted ; if the 
temperature rises too high, the colour changes to a dirty 
greyish brown, owing to the reduction of the manganic 
acid. 

This pigment is also made by heating 14 parts of man- 
ganic oxide, 80 parts of barium nitrate and 6 parts of barytes 
with access of air until the desired shade appears. The 
mass is then ground in a continuous current of water until 
it is changed into a very fine powder and nothing more is 
dissolved by the water. The best result is obtained by 
Rosenstiehl's process : 4 parts of barium hydrate, 2 parts 
of finely -powdered barium nitrate and 0'5 part of artificial 
manganese dioxide are rapidly mixed ; the mixture is moistened 



GBEEN MANGANESE PIGMENTS. 271 

and heated to a dark red heat. The fused mass is boiled 
out with water and the residue dried under a bell jar, under 
which are dishes of sulphuric acid and caustic potash, the 
former of which absorbs the water, whilst the latter keeps 
the air free from carbonic acid, which would injure the 
shade of the moist substance. 

Manganese green is amongst the pigments more recently 
discovered. On account of its high price it has found but 
limited use up to the present. It may be used in any 
vehicle, and is distinguished by great permanence. 

In regard to the preparation of this colour by Rosen- 
stiehl's method, which of all gives the best results, it should 
be observed that the shade largely depends on the quantity 
of barium hydrate used ; the greater this is the more the 
shade inclines to blue. If it is desired to produce a colour 
of a greener tone than is produced by the fusion, this can 
be accomplished by boiling for a very long time with very 
weak hydrochloric acid, which extracts a portion of the 
base from the compound and produces a deeper shade. 

BOttger's Barium Green, A beautiful green is obtained 
by the process given by Bottger, as the author has found. 
It consists of barium manganate. The process is, however, 
somewhat costly, so that the pigment would only be avail- 
able for artists' purposes. 

A solution of potassium manganate is first made by 
gradually adding 2 parts of very pure pyrolusite, finely 
powdered, to a fused mixture of 2 parts of caustic potash 
and 1 part of potassium chlorate, bringing the mass to 
a low red heat after the introduction of the whole of the 
pyrolusite, and finally extracting with water, in which the 
potassium manganate dissolves to a fine emerald green 
solution. This process is unattended by difficulty if the 
pyrolusite contains a sufficient quantity of manganese per- 
oxide and is in a sufficiently fine powder, which is most 



272 MINEEAL AND LAKE PIGMENTS. 

important. Potassium manganate is a very unstable sub- 
stance. Cold water must be used to dissolve it, and the 
solution should not long stand exposed to the air, but should 
at once be used to obtain barium manganate. 

When the solution of potassium manganate is mixed with 
a solution of a barium salt, a handsome violet precipitate 
immediately forms. This is washed with water and quickly 
ground with J to 1 part of its weight of barium hydrate. 
The mixture is heated in a copper dish with constant stirring 
to a low red heat ; the colour changes gradually to a very fine 
green. When the mass has reached the proper shade it is 
treated with cold water to remove excess of barium hydrate, 
until the washings show no trace of alkaline reaction. 

Manganous Oxide is occasionally used as a green pigment, 
especially for painting metal work ; it is obtained in the fol- 
lowing manner : Manganese sulphate solution is precipitated 
by soda solution, and the manganous carbonate obtained 
strongly ignited in a crucible. In order to avoid oxidation 
of the manganous oxide, which would readily occur in the 
process, it is necessary to prevent air from reaching the 
substance. With this object the crucible in which the heating 
is conducted is covered by another from which the bottom 
has been removed and which is filled with coal. The air 
entering the crucible on cooling must pass through the layer 
of glowing coals, by which it is deprived of oxygen. 

Manganese Blue is obtained, according to G. Bong, by 
igniting a mixture of 3 parts of quartz, 6 parts of soda ash, 
5 parts of limestone and 3 parts of manganese oxide, or of 
3 parts of quartz, 8 parts of barium nitrate and 3 parts of 
manganese oxide, air being admitted and reducing gases ex- 
cluded. All materials must be free from iron. The quantity 
of manganese oxide regulates the depth of the colour, but not 
its shade ; by increasing the soda a greener, by increasing the 
quartz a more violet, shade is obtained. 



CHAPTEE XXXV. 

COMPOUNDED GREEN PIGMENTS. 

MIXTUEES of a yellow and a blue pigment produce a green ; 
according as one or the other predominates colours are 
obtained inclining to yellow or blue. 

In some cases the mixing can be accomplished in the 
actual production of the pigments, so that a green precipitate 
is directly produced. This is, however, rarely the case ; 
generally the compound pigments are obtained by simply 
mixing the two colours. The mixing can be done either in 
the dry or the wet way ; colours in the wet state are more 
easily mixed than when dry, so that even when dry colours 
are employed the mixing is done with the addition of water. 
Although the mixture then requires a second drying, this 
method is still to be recommended, in the first place, because 
the mixing can be more quickly done in consequence of the 
greater mobility of the mass, and in the second, because the 
formation of poisonous -dust is completely avoided. 

The mixing is generally accomplished by mechanical 
arrangements. If dry colours are to be mixed, rotating 
cylinders may be used ; these are filled with the materials 
to be mixed, well closed and rotated about the axis as long 
as may be necessary. It should be observed that when colours 
which have a very different specific gravity, such as chrome 
yellow and Prussian blue, are to be mixed, the cylinders 
must be rotated for a much longer time than when colours 
of approximately equal specific gravity are to be united. 

18 

, f 

\f UNIVERSITY 

IV 




274 MINEBAL AND LAKE PIGMENTS. 

In working in the wet way, which is to be preferred, 
sufficient water is added to the colours to make a paste thin 
enough to be mixed by spatulas ; the mixture is thoroughly 
stirred and the pulp ground in ordinary mills until the 
mixture is quite uniform. Already whilst on the mills the 
pulp becomes thicker in consequence of evaporation ; when 
the grinding is finished it is spread out in thin layers so that 
it may dry as quickly as possible. This is important in the 
case of mixtures whose constituents have very different 
specific gravities, otherwise the heavier pigment may sink to 
the bottom of the paste and the mass thus lose its uniformity. 

The compound green pigments come into the market 
under most varied names, which are often entirely without 
connection with their cbemical composition. Such names 
are : mineral green, English green, oil green, green vermilion ; 
the most common are chrome green and Brunswick green. 
It should be noted that these names are more strictly applied 
to simple colours previously described. 

Chrome Green is generally made by mixing deep chrome 
yellow and Prussian blue ; it can naturally be obtained in all 
possible shades. The brightness of this already handsome 
pigment can be considerably increased by the addition of a 
small quantity of indigo carmine. In this case the mixing 
is best done by adding a solution of the indigo carmine to the 
stiff paste and again sending the mixture through the mill. 

By additions of white substances paler shades of chrome 
green are obtained ; levigated terra alba or white pipe-clay 
can be advantageously employed. 

Eisner's Chrome Green is obtained by preparing a solution 
of yellow prussiate of potash and potassium chromate and 
another solution of lead acetate and ferric chloride. The two 
liquids are mixed with vigorous stirring ; according to the 
proportions of the materials a blue or a yellow shade of 
green is produced. 



COMPOUNDED GBEEN PIGMENTS. 275 

Silk Green, Forty-one parts of lead nitrate are dissolved 
in 20 to 30 times the weight of water, the solution is boiled 
in a copper pan, and, according to the shade required, 10 to 
30 parts of fine Chinese blue are added. After well stirring, 
a solution of 10 parts of potassium bichromate and 1 part 
of nitric acid is poured into the boiling liquid, the mixture 
is again well stirred, the precipitate allowed to settle, washed 
and dried. The green pigment obtained in this way has a 
peculiar silky lustre, hence the name " silk green ". 

Natural Green is a mixture of Guignet's green with picric 
acid ; it is used in making artificial flowers instead of emerald 
green. 

Non-arsenical Green was proposed as a substitute for 
emerald green, which, however, it does not equal in bril- 
liance. It is obtained by mixing copper blue (basic copper 
carbonate) with chrome yellow, chalk and ferric oxide, in 
somewhat variable proportions. It usually contains 80 to 82 
per cent, of the copper compound and 13 to 15 per cent, of 
chrome yellow. 

In making mixtures of pigments it is to be remembered 
that chemical actions may occur in the mixture ; colours 
which act upon one another should not be mixed : the mixture 
would contain the cause of its own destruction. For example, 
a lead pigment should not be mixed with one containing 
sulphur in the form of a sulphide or sulphate. By following 
this important rule the number of pigments which may be 
mixed together is considerably diminished ; the advantage is 
that durable colours are produced, which do not change in a 
short time to an unrecognisable shade. 



CHAPTEK XXXVI. 

VIOLET MINERAL PIGMENTS. 

Chromic Chloride. The violet modification of chromic 
chloride is a compound which is little used, but deserves the 
highest attention on account of its durability and beauty of 
shade. Up to the present chromic chloride has been almost 
exclusively used for colouring paper hangings under the 
name of chrome bronze, which it has received in consequence 
of its property of imparting a peculiar metallic lustre to 
paper upon which it is rubbed. It is also possible to fix this 
substance upon fabrics, which thus acquire a similar metallic 
lustre. 

Pure chromic chloride forms beautiful peach-blossom 
coloured scales, which can only be obtained by treating 
chromium oxide in a certain manner with chlorine. The 
pure substance is practically insoluble in water, but if the 
water contains but a trace of chromous chloride the chromic 
chloride readily dissolves to a green solution. In the prepara- 
tion of this compound it is necessary to avoid the least trace 
of chromous chloride, otherwise the product will begin to 
change when it comes into contact with moist air. 

Pure chromic chloride is obtained by proceeding according 
to the method first given by Wohler : the simple apparatus 
necessary is depicted in Fig. 29. Pure chromic oxide is 
first made by any suitable process ; this is made into a paste 
with charcoal, starch paste and water, and formed into small 
balls which are heated in a crucible to a white heat. The 



VIOLET MINEKAL PIGMENTS. 



277 



residual intimate mixture of chromic oxide and carbon is 
loosely placed in a crucible which stands in a furnace ; a 
porcelain tube is cemented into the bottom of the crucible, 
it passes through the ashpit to connect with a chlorine 
apparatus. Upon the crucible another larger one is placed, 
which has a small opening in the bottom and serves as a 
receiver for the sublimed chromic chloride. The heating and 
current of chlorine are started at the same time, the fire is 
regulated so that the lower crucible glows brightly. This 




FIG. 29. 

is continued for an hour, during which a rapid current of 
chlorine must pass into the crucible ; the fire is then removed 
and a slow current of chlorine passed through the apparatus 
until it is quite cold. 

If the operation has succeeded the lower crucible is found 
empty and the interior of the upper covered with the mag- 
nificent peach-blossom crystals of chromic chloride. A small 
quantity of the chromic chloride is tested as to its behaviour 
towards water ; if it remains unaltered the whole may be 



278 MINEEAL AND LAKE PIGMENTS. 

washed, but if it dissolves to a green solution it must be 
again ignited in a current of chlorine. The washing is 
necessary to remove small quantities of aluminium chloride 
formed by the action of chlorine on the clay of the crucible. 

Manganese Violet, or Nuremberg Violet, consists of man- 
ganic phosphate. It is obtained by fusing glacial phosphoric 
acid with pure pyrolusite, boiling the melt with ammonium 
carbonate, filtering the solution, evaporating to dryness, and 
again fusing the residue. After boiling with water this 
forms a handsome violet powder ; it is suitable for use as 
a very durable artists' colour. 

Shades of this pigment inclining to blue are obtained 
when a ferric compound is added in the first fusion ; the 
more iron ore is used the more intense is the blue tone. 

Tin Violet or Mineral Lake is obtained as a violet mass by 
igniting an intimate mixture of 100 parts of tin dioxide with 
2 parts of chromic oxide. This pigment is completely per- 
manent in air ; it is suitable for printing paper hangings and 
for colouring faience. 

Copper Violet, Guyard's Violet. According to J. Depierre, 
this pigment is obtained by precipitating a solution of copper 
ammonium sulphate with potassium ferrocyanide, washing 
and drying the precipitate and heating in a porcelain dish. 
At 170 C. cyanogen and ammonia are evolved, the mass 
takes up oxygen and becomes violet ; it then resists the action 
of dilute acids and alkalis, and as a pigment has great cover- 
ing power. On heating to 200 C. it turns blue, and at 240 
to 250 C. becomes greenish. 



CHAPTEK XXXVII. 

BROWN MINERAL PIGMENTS. 

Lead Brown, When red lead is treated with nitric acid, 
lead monoxide is dissolved, whilst deep brown lead peroxide 
remains. When the action of the nitric acid is finished the 
residue is well washed and dried. Lead peroxide is now 
extensively used for lucifer matches ; it readily gives up 
oxygen on heating, and thus accelerates the ignition of the 
composition. Brown match-heads owe their colour to lead 
peroxide. 

Manganese Brown consists of manganic oxide ; it occurs 
in nature, but seldom in a state of sufficient purity to be used 
as a pigment ; it is, therefore, generally made artificially. 
The process is very simple. A solution of manganese sul- 
phate is precipitated by caustic soda solution, the manganous 
hydroxide produced is quickly changed by the action of the 
air into manganic hydroxide. The colour is most rapidly 
developed when the precipitate is spread out in thin layers ; 
it is well washed after it has changed to brow y n. 

Pyrolusite Brown is manganese peroxide in a state of fine 
division. It can be made from the residual liquors of the 
preparation of chlorine by adding sodium hypochlorite. A 
brown precipitate results, which is kept in contact with the 
liquid until it no longer changes in colour. It is then washed 
with water slightly acidified with sulphuric acid, and finally 
with pure water. When finely powdered, manganese peroxide 
has a very fine brown colour. It is quite unaltered by the 



280 MINEEAL AND LAKE PIGMENTS. 

air and thus, in consideration of its cheapness, deserves a 
much greater use than it has yet found. 

Prussian Brown, When Prussian blue is heated in air 
it soon begins to glow, and is converted into a brown mass, 
the shade of which depends on the amount of impurity in 
the Prussian blue. It is desirable to use Prussian blue quite 
free from impurities, so that a brown of the same shade is 
always obtained. The brown obtained can be shaded by 
additions of indifferent substances. 

Iron Brown. By igniting a mixture of 100 parts of finely 
levigated natural yellow ochre with 5 parts of common salt 
a brown product is obtained, which is pale or deep accord- 
ing to the temperature used. It is a cheap and durable 
colour. 

Copper Brown. According to Cartheuser solutions of 
2 parts of copper sulphate and 1 part of Epsom salts are 
mixed, and a strong solution of potassium carbonate gradually 
added so long as a precipitate forms. The precipitate is 
washed, dried and ignited, when copper brown is obtained 
as a brown powder of great permanence in air. 

According to Schre^er solutions of 2 parts of copper 
sulphate, 2 parts of alum and 0*5 to 1 part of ferrous 
sulphate are .mixed, potassium carbonate solution added, 
the precipitate dried and ignited. The redness of the shade 
is proportional to the quantity of ferrous sulphate. 

Hatchett Brown is copper potassium ferrocyanide. It is 
obtained by precipitating a soluble copper salt with potassium 
ferrocyanide. Copper salts behave towards potassium ferro- 
cyanide in the same manner as iron salts ; different com- 
pounds result according as the ferrocyanide or the copper 
salt is in excess. Hatchett brown is somewhat largely 
used for painting wood. 

Chrome Brown. When solutions of potassium chromate 
and a copper salt are mixed a precipitate of the composition 



BROWN MINERAL PIGMENTS. 281 

CuCr0 4 .2CuO.2H.,0 is obtained. On drying it acquires a 
fine brown colour ; it forms a very stable pigment. 

' Chrome brown can also be made by dissolving 10 parts 
of potassium bichromate in 20 parts of water, heating to 
boiling, adding 13 '5 parts of solid copper chloride and then 
gradually a boiling solution of 10 parts of soda in 20 parts 
of water, until effervescence no longer takes place. On 
cooling, chrome brown separates as a soft brown precipitate. 

Cobalt Brown is a very durable compound pigment of 
a pleasing shade. It is obtained in various hues by adding 
ferric oxide to the mixture of alumina and a cobalt salt used 
for making cobalt blue. The process may also be carried out 
by igniting ammonia alum with cobalt sulphate and ferrous 
sulphate. The temperature required in this case to obtain 
a bright colour is very high, and must be maintained for 
a long time to decompose the whole of the ferrous sulphate. 

The pigment can be obtained at a far lower temperature 
when ferric chloride is used in place of ferrous sulphate. 
The mixture is made by grinding together 5 parts of cobalt 
hydroxide and 25 parts of ammonia alum, then adding a 
solution of ferric chloride, rapidly drying the whole to a 
powder, and whilst still hot filling it into the crucible in 
which the ignition is performed. When small quantities 
of ferric chloride are used chocolate brown shades inclining 
to violet are produced ; the larger the quantity of the iron 
compound the more pure is the brown. 

It has been stated that it is important, in the preparation 
of cobalt pigments, to prevent the entry of fire gases to the 
heated mixture ; the reducing action of the gases would 
materially damage the beauty of the colour. It has been 
proposed to place a small quantity of mercuric oxide at the 
bottom of the crucible. This would decompose on heating, 
and create an atmosphere of pure oxygen in the crucible. 
Apart from the cost of the mercuric oxide, which is con- 



282 MINERAL AND LAKE PIGMENTS. 

siderable, it would only be effective at the commencement 
of the operation, since it is completely decomposed at a low 
red heat. The author has found an addition of pyrolusite 
considerably more effective. This may be applied by spread- 
ing a small quantity, about 5 per cent, of the mixture to be 
heated, on the bottom of the crucible and covering it with 
powdered glass, upon which the colour mixture is placed. 
An alternative course is to place the crucible in a second, 
filling the space between the crucibles with powdered pyro- 
lusite. At a strong red heat oxygen is slowly evolved from 
the pyrolusite ; the entry of fire gases into the crucible is 
thus prevented. In using the first method the crucible is 
frequently spoilt, whilst in the second method the sur- 
rounding pyrolusite protects it, so that it can be used again. 



CHAPTEE XXXVIII. 

BROWN DECOMPOSITION PRODUCTS. 

Humins, Wood decomposes, like many other substances 
of organic origin, to form compounds of a deep brown colour, 
which are known as humins, on account of their occurrence 
in the humus of tilled soil. The numerous compounds found 
in humus have all a deep brown colour, which, together with 
their great stability, makes them very suitable for use as 
pigments. 

By various methods substances can be made so rich in 
humins that they can be used as pigments. For this purpose 
sugar, starch, young plant fibres and beet-sugar molasses 
may be used, which are converted into humins with great 
rapidity when they are heated with water. In this way the 
author has obtained handsome colours from sawdust. These 
humins are most easily made by the following process : 
Thick beet-sugar molasses are cautiously heated with 5 per 
cent, of caustic soda in a very capacious iron pan. The 
already dark mass soon becomes quite black when seen in 
thick layers, and evolves a considerable quantity of gas ; if the 
heating is too rapid the soft mass may boil over out of a very 
large vessel. When the evolution of gas diminishes the heat 
is increased and the mass frequently stirred. With some 
practice it is possible to tell when the action is finished 
from the smell, which is at first sweetish but later of a 
characteristic nature. At first tests should be repeatedly 
taken from the mass ; these are considerably diluted with 
water until the liquid begins to be transparent. When two 



284 MINEBAL AND LAKE PIGMENTS. 

tests taken at an interval show no difference of colour the 
heating is stopped. The whole mass is then poured into 
water in order to dilute the alkali, so that it will not destroy 
the strainer ; the soft mass, which in the wet state appears 
quite black, is washed with water until the washings are 
neutral. The humin brown made in this way, when ground 
with oil or gum solution, produces a very handsome brown 
of great covering power and warmth of shade, also distin- 
guished by complete indifference towards chemical reagents. 

The pigments we have designated humins contain a very 
large quantity of carbon, to which they owe their dark 
colour. When peat or lignite is treated with caustic soda in a 
similar manner, good shades of brown are obtained ; they are, 
however, surpassed in beauty by the brown from molasses. 
A handsome but costly brown is obtained when crude 
spirits of wine are heated with fuming sulphuric acid. Equal 
volumes of alcohol and sulphuric acid are used ; the mixture 
is heated in a retort connected to a condenser, which is 
required on account of the combustible vapours evolved from 
the mixture. When the mass is quite black the heat is 
withdrawn ; the residue is diluted with water, and soda solu- 
tion added so long as effervescence occurs. On filtering a 
very soft brown powder is left, which forms a very handsome 
and durable pigment. 

Bistre. When soot is produced at a very low tempera- 
ture it is very lustrous, and, in addition to carbon, contains 
a notable quantity of the products of dry distillation. When 
this soot is powdered and treated with water the latter sub- 
stances are dissolved ; boiling water should be used. When 
water is no longer coloured the soot is suspended in a large 
quantity of water and subjected to a process of levigation. 
The fine powder obtained by repeated levigation has, when 
dry, an ugly brown colour, but when ground it acquires an 
extremely warm shade. 



CHAPTEK XXXIX. 

BLACK PIGMENTS. 

CABBON occurs in nature in many different forms the dia- 
mond, graphite (black lead), and purified lamp black are, from 
the chemical point of view, one and the same substance, 
namely, carbon. In colour making the non-crystalline form of 
carbon, which is pure black, is alone used ; almost all black pig- 
ments used in painting are composed of tolerably pure carbon. 
In whatever way and from whatever materials carbon is made 
for use as a pigment, the manufacturer must always endea- 
vour to obtain it as pure as possible, and in a condition of 
the finest division ; the depth of colour and the covering 
power depend on these two conditions. Carbon only shows 
a pure black colour when it is pure ; if it contains relatively 
small quantities of impurities, it has a more or less brown 
shade. 

At first sight it appears to be a very simple matter to 
obtain black pigments from organic materials ; it is simply 
necessary to expose them in the absence of air to a tempera- 
ture high enough to decompose them, carbon is then left as 
a residue. In spite of the apparent simplicity of this opera- 
tion there are many difficulties. The preparation of good 
black pigments demands considerable practice. 

There are two methods by which carbon can be made for 
use as a pigment. One has been indicated, the heating of 
organic materials in the absence of air, i.e., their dry distilla- 
tion ; the second, which produces the best qualities of 



286 MINEEAL AND LAKE PIGMENTS. 

black, consists in burning substances very rich in carbon 
with a very small supply of air. The product of this pro- 
cess, commonly known as soot, is very different in external 
appearance from carbon made by dry distillation. The latter 
forms an easily powdered mass, which only acquires lustre 
by grinding with oil, whilst soot consists generally of very 
light flocks, which, in consequence of their greasy nature, 
exhibit a peculiar velvety lustre. 

In commerce there are many qualities of black, the names 
of which are generally chosen quite arbitrarily. The so-called 
ivory black is an example of this. At some period it was ob- 
served that ivory produced a very fine black when carbonised ; 
this black was made for a long time from refuse ivory. When 
it was found that a similar black could be made quite as 
well from much cheaper materials, it was no longer made 
from ivory. The name, however, remains in trade to the 
present day : it has come to be regarded as a description of 
quality. By the name of ivory black a fine black pigment is 
understood ; it is immaterial to the consumer whether it is 
made from ivory or not : to him the quality of the pigment, 
but not its source, is important. 

According to the method of production, the black pig- 
ments can be divided into charcoal blacks and soot blacks. 

CHAECOAL BLACKS. 

This term is applied to the pigments obtained by heating 
organic substances in the absence of air. The charcoal 
blacks have two valuable properties : they are easily made, 
and they exhibit a pure black shade which it is far more 
difficult to obtain with the soot blacks. It is very difficult 
to convert these pigments into the requisite state of fine 
division. They cannot be levigated on account of their low 
specific gravity, which causes them to settle very slowly in 
water. It only remains to convert the charcoal into a very 



BLACK PIGMENTS. 287 

soft powder by grinding, but even then the pigment has but 
little covering power, because it is not possible to destroy the 
organic structure of the substance carbonised. When the 
finest charcoal powder, made by carbonising any organic 
substance, is examined under a powerful microscope the 
structure of the particles is at once recognised, so that it 
is almost always possible to decide whether the charcoal is 
of plant or animal origin. A very careful examination may 
show what particular part of the plant has been carbonised. 
If a manufacturer succeeded in producing by direct carbonisa- 
tion a black with the covering power of a soot black, that 
black would soon be the only one found in commerce. 

The charcoal blacks come on the market under various 
names ivory black, bone black, vine black, Frankfort black, 
Paris black, etc. The ivory blacks are the best quality, but 
recently true vine black, i.e., made from grape residues, has 
come into use. 

True Charcoal Black In charcoal-burning wood is burned 
in great piles with restricted air supply ; the greater part of 
the carbon contained in the wood remains behind as charcoal, 
which is used for fuel. The charcoal made from hard wood, 
such as maple or beech, is not well suited for pigments ; the 
lighter and more porous the wood, the more pure is generally 
the shade of the charcoal, and the more easily it can be 
ground. When, therefore, charcoal is to be used as a pig- 
ment it should be made from wood of a porous nature ; the 
wood of the lime, black-alder and spindle tree is particularly 
adapted for this purpose. Spent tan bark is a very cheap, 
and at the same time suitable material for this purpose. It 
generally consists of oak bark, and has lost the greater part 
of the salts it contained by long contact with water ; also the 
woody substance of the bark has suffered a change which is 
favourable to carbonisation. This is especially the case when 
the bark has been stored for some time ; a considerable quantity 



288 MINERAL AND LAKE PIGMENTS. 

of the humins previously mentioned has formed in it, and these 
are easily decomposed at a low temperature. 

The charcoal, however obtained, is ground to a soft powder. 
When it has reached the proper degree of fineness it should 
be repeatedly washed with water to remove the salts. When 
instead of pure water a very dilute acid, such as hydrochloric, 
is used, practically all the salts dissolve, and the residue con- 
sists of nearly pure carbon. 

Vine Black, In wine-producing countries a good and 
cheap black can be made from the residues of the wine 
manufacture ; this is the so-called vine black. Either the 
pressed grapes or the lees separated in the fermenting vessels 
may be used. 

Vine Black from Wine Lees. The lees always contain a 
considerable quantity of liquid, they must be thoroughly 
dried before they are carbonised ; this is most simply done 
by spreading out the pasty mass in a thin layer, and exposing 
it to a temperature of about 100 to 120 C. In drying 
the volume considerably diminishes. The lees are changed 
into a brown mass which is easily powdered ; it is packed into 
barrels while still warm, and can be kept for a considerable time 
Without alteration. Fresh lees can be kept but a short time ; 
they are quickly destroyed by a rapid fermentation. 

The dried lees are carbonised in iron tubes, protected 
from the fire by a thin coating of clay mixed with chopped 
hair so that the coat more readily adheres to the iron. 
Old stove pipes or cast-iron gas or water pipes may be 
used. The tubes are about one metre long ; they are closed 
with well fitting covers, in one of which is a small opening 
through which the gases produced on heating can escape. 
These covers are fastened on air-tight with clay. The pro- 
cess begins by affixing the unperforated lid, and packing 
with a wooden rammer the dry lees as tightly as possible 
into the tube ; the second lid is then luted in place. The 



BLACK PIGMENTS. 289 

tubes are placed near one another in a suitable furnace and 
first slowly heated at the back, i.e., the end closed by the 
unperforated cover. At the commencement of the operation 
heat must be gradually applied. With too large a fire the 
products of dry distillation might be evolved in sufficient 
quantity to force the covers from the tube ; air would then 
enter and the contents be burnt. When the hinder part of 
the tube is red hot, the heating is conducted forward, and 
finally the whole length of the tube is brought to a good red 
heat. The termination of the operation is shown by the- 
disappearance of the pointed flame of the products of distilla- 
tion which protrudes from the opening in the cover of the 
tube. When this occurs, the fires are drawn ; the tubes are 
left to cool in the furnace until they can be taken out by the 
hands protected by wet cloths. The covers are at once taken 
off and the hot contents emptied into a large tub filled with 
water, which is thus soon raised almost to boiling point. The 
time required for washing is thus considerably shortened. 
The charcoal falls from the tubes in lumps, which soon fall 
to a fine powder in the water. Wine lees consist of a mixture 
of yeast cells and small crystals of tartar (a mixture of the 
tartrates of potassium and calcium). On ignition these salts 
are converted into carbonates, by which the particles of carbon 
are united. Potassium carbonate is very readily soluble in 
water ; in contact with the warm water it dissolves in a very 
short time, the carbon particles then fall apart. 

When the charcoal has settled to the bottom of the 
vessel the liquid is drawn off. It can be utilised in a colour 
works, since it is a fairly strong solution of potassium 
carbonate. The charcoal is mixed with calcium carbonate, 
arising from the decomposition of the calcium tartrate, and 
with the other insoluble salts which yeast ashes contain in 
some quantity. These salts would prevent the proper 

grinding of the charcoal ; they are therefore removed by 

19 



290 MINERAL AND LAKE PIGMENTS. 

treatment with hydrochloric acid after the water has been 
drawn off as completely as possible. A small quantity of 
hydrochloric acid, diluted with an equal volume of water, is 
poured over the charcoal, an effervescence of carbonic acid 
follows, the easily soluble calcium chloride is formed and the 
remaining salts are also dissolved. 

The charcoal made in this way is very pure. After washing, 
grinding and drying, it forms a pigment whose shade leaves 
nothing to be desired. The drying must be conducted at 
a low temperature ; charcoal in such a fine state of division 
is very easily inflammable. 

Vine Black from Pressed Grapes. The material contains the 
stems and pressed remains of the grapes after the must has 
been expressed. After-wine, spirits or vinegar can be obtained 
before it is used for vine black. The process is exactly the 
same as for wine lees. The charcoal taken from the tubes has 
rather more coherence than that made from lees and must 
be ground. The black is a very useful pigment, but is inferior 
to the black from lees. In regard to the latter, it should be 
stated that it is a particularly good black pigment, and when 
finely ground is as well suited for the preparation of the 
finest blacks, such as are used in copper-plate printing, as the 
far more costly soot black. 

The black from grape residues, or from grapes themselves 
(the poor grapes removed in pruning the vine are used), can 
be employed with advantage for many purposes for which the 
best black is not necessary. The greater number of the pig- 
ments sold under the names of Frankfort and Paris black 
consist of this substance. 

Bone Black or Ivory Black. Bone black, for of ivory black 
only the name now exists, is distinguished by a peculiar 
quality which it owes to the structure of the raw material 
from which it is made. This raw material is animal bones, 
which largely consist of incombustible materials (bone ash) ; 



BLACK PIGMENTS. 291 

these form a delicate framework whose interstices are filled 
by organic matter. When bones are carbonised, the carbon 
resulting from the decomposition of the organic matter is 
deposited on the incombustible framework of bone ash and 
thus acquires a very great surface. 

Carbon is well known to possess very powerful absorptive 
properties, which are especially developed in bone black, in 
consequence of its fine division. The use of granular bone 
black (known as " char ") in sugar works, and generally for 
decolourising liquids, is due to the peculiar state of division 
of the charcoal. For these purposes bone black is used in 
enormous quantities ; it is made in special works. It does 
not fall within the scope of this work to give a detailed 
description of the manufacture of bone black, and we must 
here restrict ourselves to matters of special interest to the 
colour maker. 

The bones are coarsely powdered and freed from fat by 
boiling ; they are then generally carbonised in iron retorts, 
a number of which are built in a furnace in a vertical posi- 
tion ; a valve at the bottom serves to empty the retorts. In 
a well-arranged apparatus the products of the dry distillation 
of the bones, chiefly consisting of ammonium carbonate, are 
collected. The retorts are carefully closed so that air cannot 
enter, otherwise a portion of the carbon would burn and the 
black would acquire a grey shade instead of the pure black 
which it should possess. 

Bone black makers who work for sugar factories car- 
bonise by preference the densest bones ; these produce a 
black of the most powerful decolourising properties. This 
property is of no advantage to the colour maker, who 
simply requires a very dense black. The bones of young 
animals, and especially certain bones, contain a larger pro- 
portion of cartilage than the hollow bones which produce 
the best " char ". In making bone black to be used as a 



292 MINEEAL AND LAKE PIGMENTS. 

pigment just those bones should be chosen which are of 
less value for " char ". 

On the small scale, bone black can be made by carbonising 
in crucibles ; these hold about 16 kilogrammes of bones, they 
have a projecting rim, so that when one is placed upon the 
other it serves as a lid for the first. Piles of these pots are 
formed, the top one being covered by a well-fitting lid. The 
piles are heated in a furnace in such a manner that the flames 
can freely circulate between them. At first a small fire is 
applied ; as soon as dry distillation of the bones begins, which 
is recognised by pale white flames at the rims of the crucibles, 
the fire is damped, because the burning products of distillation 
produce so much heat that the crucibles are soon at a good red 
heat. 

When the flames disappear the fire is maintained for about 
fifteen minutes longer, and when the crucibles have cooled to 
some extent they are taken out of the furnace and immediately 
emptied into a sheet-iron cylinder, in which the charcoal re- 
mains until quite cold. If the black comes in contact with air 
whilst still hot a portion of the carbon burns and the product 
has little value either as a pigment or for decolourising pur- 
poses. When quite cold the black is ground and levigated. 
The bone ash it contains makes these operations easy. Bone 
black contains at the most 12 to 13 per cent, of carbon ; the 
remainder consists of bone ash and water absorbed by the 
hygroscopic carbon from the air. 

Bone black which has been partially burnt in the process 
has a greyish tinge ; it may also have an ugly shade of brown : 
this is the case when it has not been heated to a sufficiently high 
temperature, so that it still contains some quantity of organic 
matter. Such black can be made usable by again heating, 
but it would always be desirable to test a small portion before 
the whole was ground and levigated ; a uniform heating of the 
finely-pow 7 dered black would be attended with difficulty. 



BLACK PIGMENTS. 293 

Finely ground bone black has numerous uses as a pigment ; 
by a simple process it can be converted into almost pure carbon, 
which can be used as an excellent black pigment for all pur- 
poses for which blacks are employed. Bone black consists of 
bone ash upon which fine particles of carbon are deposited. 
Bone ash is easily soluble in hydrochloric acid ; if finely ground 
bone black is treated with this acid and the residue washed 
with water until the washings are neutral, a residue of ex- 
tremely soft and pure carbon is obtained, which has a deep 
black colour and, in consequence of its fine division, very 
great covering power. 



CHAPTEE XL. 

MANUFACTURE OF SOOT PIGMENTS. 

THE soot which is formed in the incomplete combustion of 
organic substances containing a large proportion of carbon 
is a mixture of different substances, of which carbon is the 
chief ; in addition to this, we find in soot almost all the 
products which result by the dry distillation of the sub- 
stance from which it is formed. Soot from hard wood 
contains different compounds to soot from soft wood ; again, 
rosin soot is of a different nature to that obtained in the 
incomplete combustion of fats. We can thus define soot as 
very finely divided carbon, mixed with the products of dry 
distillation. 

The soot of hard woods, which contain little or no resin, 
has a deep black, though dull hue ; it is a gritty powder, and 
has little value as a pigment. Wood containing much resin, 
such as the wood of the pine, rosin, fish oil, asphaltum, in 
short, all bodies which are at the same time easily com- 
bustible and rich in carbon, produce on the contrary a 
handsome, glistening soot, which forms a valuable pigment. 

In addition to this difference in composition, soot from 
different sources differs also in the size of its particles. Soot 
forms light flocks, which adhere to projections in the flues 
through which the products of combustion pass ; the larger 
the flocks the sooner they are deposited, the smaller they 
are the longer they remain suspended. The finest particles 



MANUFACTUEE OF SOOT PIGMENTS. 295 

of soot are called "flying soot," which is very highly prized 
on account of its fine division. 

The manufacture of soot blacks is a very important 
industry : black printing ink, which is used in such enormous 
quantities, is made from a soot black ; in addition, all the 
best black paints and lacquers. Soot black was formerly 
made in the most primitive manner, and is still, to some 
extent, as will be seen from the account of the manufacture 
of rosin black. The process used for printing blacks is 
more rational, but is still capable of great improvements. 
The principles of a rational manufacture of soot blacks will 
be briefly indicated. 

The Manufacture of Soot Blacks on the Large Scale, 
The principle of the arrangements necessary for making soot 
blacks is very simple. An apparatus is required in which 
substances very rich in carbon can be burnt at the lowest 
possible temperature ; this apparatus must be connected with 
suitable arrangements for retaining the soot carried away with 
the products of combustion. The soot works as at present 
arranged are developments of the crude arrangements still 
used in districts in which there is an abundant supply of 
pine wood containing much resin. The apparatus used in 
such localities consists of a low masonry flue connected with 
a long pipe built of wooden boards. In order to give this 
pipe a rough surface upon which the soot can readily 
deposit, it is lined in several places with coarse linen, to the 
projecting fibres of which the soot adheres. In the flue the 
very resiniferous wood is burnt, especially the roots of pines, 
which are very rich in resin ; these burn with an unrestricted 
air-supply with a bright flame, but when the air is restricted, 
they give off a large quantity of a very heavy smoke. The 
operation is commenced by first making a good fire of dry 
split wood, the object of which is to heat the flue and thus 
prevent the deposition of soot in it in the later part of the 



296 MINERAL AND LAKE PIGMENTS. 

process. If a deposit of soot formed in the flue it might take 
fire, the fire would then spread further into the wooden 
flue and a considerable loss of soot would occur. When the 
stone-work has become so hot that deposition of soot in it is 
no longer to be feared, the materials are introduced from 
which the soot is to be made. As has been said, pine roots are 
generally used, the chips of pine wood are also used in some 
districts, and in general such combustible materials as pro- 
duce a large quantity of smoke. 

The combustion in the flue must be conducted with care, 
it should proceed at the lowest possible temperature, but this 
should not sink below a certain minimum. If the fire is too 
strong the greater portion of the carbon is burnt, which 
would otherwise be obtained as soot; the yield of soot is 
very small, and generally only " dust soot " would be ob- 
tained, without any quantity of " flying soot ". On the 
other hand, if too little air enters, so that the combustion 
takes place at too low a temperature, a large yield of soot is 
obtained, but it is of poor quality. Soot obtained by com- 
bustion at too low temperatures has not a pure black but a 
perceptibly brown colour; it has not the flocculent nature of 
the best product, a small weight of which occupies a very 
large volume, but is greasy and has a high specific gravity. 
The lower the temperature used in producing soot black, the 
further is the chemical change removed from that of ordinary 
combustion, and the nearer to that of dry distillation. Soot 
produced with an insufficient supply of air contains a 
considerable quantity of liquid and solid products of dry 
distillation, which give it a brown colour and the above- 
mentioned greasy nature. 

In many soot works the air supply is regulated in a most 
primitive manner : the workman places against the openings 
by which air is admitted to the burning materials a larger or 
smaller number of bricks according as the combustion appears 



MANUFACTURE OF SOOT PIGMENTS. 297 

to be too rapid or too slow. Unfortunately there is no clear 
sign to indicate the temperature most favourable for the pro- 
duction of the greatest quantity of soot, the appearance of the 
flame alone can be taken as a guide. If the flame appears 
pure white and very luminous and shows at its end no thick 
black smoke the combustion is very complete, and but a very 
small yield of soot is to be expected. On the other hand, if 
the flame continually threatens to go out the air supply is 
too small and a liberal amount of soot will be formed, but 
mixed with a large quantity of the products of dry distillation. 
As far as it can be described in words the flame should appear 
as follows : The colour should be a murky red, similar to that 
of the flame of a bad tallow candle ; in shape the flame should 
be a long drawn-out tongue, from the point of which a thick 
black smoke is clearly seen. The soot deposits in the long 
board flues in the form of flocks or dust. The properties of 
the soot which is deposited in the different parts of the long 
flue vary according to the distance from the place of com- 
bustion, near to which a good black soot is deposited, re- 
quiring, however, long grinding with oil or gum solution to 
form a good paint. At a greater distance the soft and very 
fine " flying soot " is deposited ; it is the most pure black 
and is regarded as the best quality. The soot deposited farther 
on is more and more brown, and has a more greasy nature the 
greater the distance from the place of combustion. 

The flue for the reception of the soot must be made so long 
that hardly any smoke is perceptible at the end, which it is 
convenient to connect with a chimney with a good draught, 
regulated by a damper. In this case the combustion and the 
the speed at which the products enter the flues are twice 
regulated. 

The material of which the flues are constructed through 
which the products of combustion are led influences the 
manner in which the soot is deposited ; the portions nearest 



298 MINEBAL AND LAKE PIGMENTS. 

to the grate must be of stonework, but when the smoke has 
cooled to a certain extent different materials can be used in 
addition to wood ; flues made of sacking stretched over laths 
are used. The fine flying soot adheres to the roughnesses of 
the coarse sacking, and is easily loosened by striking the wall 
of the flue. 

As we have said, the arrangements of most soot works are 
very primitive ; they obtain only a fraction of the quantity of 
soot which they could produce by a proper regulation of the 
draught in the combustion chamber and in the flue in which 
the soot deposits. We shall now briefly describe the condi- 
tions which should be regarded in constructing a soot works ; 
these conditions have not been found to be observed in any 
works we have seen, in this branch of chemical industry 
so-called practical experience is alone regarded, and conse- 
quently the " practical" soot burner loses large sums, literally 
up the chimney. A black works can be properly designed by 
any one who knows sufficient chemistry to understand the 
process of combustion. Of this most educated men have some 
understanding, but the practical soot burner appears to have 
but hazy ideas on the subject, for one occasionally finds the 
arrangements for making soot in direct opposition to the 
proper disposition. 

For the sake of the uniformity of the product and of safety 
against fire, the flues in which the soot is deposited should 
be entirely of masonry ; the joints of the bricks should be 
smoothly cemented over, so that soot cannot collect in quan- 
tity in them. The end of this flue should be connected with 
a high chimney provided at the top with a well-fitting damper,, 
so that the draught in flues and chimney can be regulated at will 
or completely stopped. Such an arrangement, perhaps some- 
what costly, has many important advantages : it is fireproof, 
and when once warm remains so for a long time, since bricks 
are bad conductors of heat. When the flue is hot no water 



MANUFACTUBE OF SOOT PIGMENTS. 299 

condenses in it ; all the water formed in the combustion 
remains in the form of vapour, and is carried away by the 
chimney. A further advantage is that it is not often necessary 
to enter the flue to collect the soot ; burning may be continued 
for a long time, and a large quantity of soot taken at once 
from the flues or soot chambers. 

The soot collects on the walls of the chambers in flocks, 
which finally become so heavy that they fall off and collect on 
the floor. The entry to the soot chambers should be through 
a single iron door, tightly closed whilst the chambers are in use, 
and cemented round. If this door does not fit air-tight the 
combustion cannot be exactly regulated by the damper on the 
chimney. The soot is removed from the chambers by a 
workman, who sweeps off with a soft brush into a sheet-iron 
vessel the soot adhering to the walls and lying on the ground. 
It is extremely important that soot and nothing else should 
be collected ; the brush used for loosening the soot should be 
so soft that it does not rub off mortar from the brickwork ; 
the workman's shoes should be provided with felt soles, so 
that no particles are loosened from the floor of the chamber 
and mixed with the soot. The admixture of the smallest 
quantity of sand would be extremely harmful in the ensuing 
grinding of the soot ; the mills would be damaged. 

The flues for collecting the soot in well arranged works 
are very similar, but the apparatus used for burning the 
materials which produce the soot varies greatly according 
to the nature of the material. Some quantity of the soot 
black used in the arts is still made by burning pine roots and 
chips, but for the finer qualities American rosin is largely 
employed. Earth-wax or ozokerite and the hydrocarbons 
obtained from petroleum and in the distillation of shale are 
materials frequently used in making soot blacks of very good 
quality. For the finest qualities, such as are used for fine 
printing inks, copper-plate inks and black lacquers, soot 



300 



MINEKAL AND LAKE PIGMENTS. 



obtained by burning fish oils or cheap fatty oils is most 
commonly used. The great differences in the physical nature 
of these materials demand the use of different apparatus for 
their combustion. The space allotted to soot blacks in this 
work would be far exceeded if the construction of all the 
forms of apparatus were described ; the most important only 
will be given. 

When rosin is the raw material, the combustion can be 
conducted in flat spoon-shaped vessels placed before a narrow 
opening into the flue. Fig. 30 shows an arrangement very 




FIG. 30. 

successful in practice. The spoon-shaped iron vessel, G, 
stands in a second, G x , which is filled with water : this pre- 
vents the fused rosin from becoming too hot. If the 
temperature in G rose too high, dry distillation would take 
place along with the combustion of the rosin, and the soot 
would be largely contaminated by the products of this dis- 
tillation. This may proceed so far that in place of the fine 
flocks of soot a greasy mass is deposited in the flues, con- 



MANUFACTUBE OF SOOT PIGMENTS. 301 

sisting of a mixture of soot with products of distillation, from 
which the soot could be obtained only with great difficulty. 
The soot and gaseous products of combustion pass through 
the opening, O, into the flues, B R. This opening is only 
several centimetres wide, but is nearly as long as the com- 
bustion vessel. Above the vessel, G, is a movable iron 
cover, D, in which are slides by which the air supply is 
regulated. The cover is only taken off when fresh material! 
is introduced. 

The air supply cannot be sufficiently regulated by the 
slides in the cover ; these must be used in conjunction with 
the chimney damper. The combustion is observed through a 
thick glass plate let into the cover. At the commencement 
of the operation slides and damper are completely opened. 
When the chimney is seen to emit a thick black smoke the 
flues are filled with the products of combustion, and the 
speed with which they pass through the flues must be regu- 
lated. The current of air is decreased until only a little 
visible smoke escapes from the chimney, and the flame is no 
longer white, but a murky red. It should be noted that the 
first soot obtained from a new works is never of the best 
quality, the production of this is only gradually attained. 
The cause of this is that the new masonry is damp and gives 
up water to the hot gases, so that they are cooled and their 
velocity disturbed ; the soot will also be damp. Thus a 
greasy soot is obtained. It is of very little use to allow the 
installation to remain unused for some months after it is 
completed ; the brickwork would only dry superficially, and 
when it was first heated the presence of the water would 
become evident. 

In order to dry the whole erection to such an extent that 
the moisture from the brickwork has no effect on the regular 
course of the operation, it is advisable to commence with 
materials of little value and to allow a portion of them to be- 



302 



MINEBAL AND LAKE PIGMENTS. 



lost by the use of a stronger draught than is generally used, 
so that the flues can be freed from all moisture as soon as 
possible. This applies equally to all kinds of arrangements 
for this purpose. 

Lamps are used to burn liquid fats and mineral oils. 
These lamps naturally have a different construction to those 
used for lighting, which are constructed with the object of 
burning all the carbon in the oil, so that the temperature 
may rise as high as possible and the carbon burn at a high 




FIG. 31. 

white heat. The lamps used in the soot black manufacture 
are designed to burn only as much carbon as is absolutely 
necessary to maintain the flame, at the same time the 
temperature of the flame must be kept low, so that no portion 
of the soot is again burnt. These lamps have flat burners 
and are enclosed in a sheet-iron casing provided with an 
air regulator, which must work very accurately, otherwise 
air will enter between the joints and the working of the 
regulator will be deceptive. So that the material to be burnt 



MANUFACTURE OF SOOT PIGMENTS. 303 

shall not be too strongly heated, which would be accompanied 
by great loss when mineral oil is used, the reservoir should 
be placed outside the iron casing which surrounds the burner. 

Fig. 31 shows the construction of a soot lamp. The flat 
burner, B, is placed in the cylindrical sheet-iron mantle, H, 
which is bent above at not too sharp an angle. The products 
of combustion are led into a chamber, K, from which they 
pass into the flues where the soot is deposited. The form of 
the upper portion of the cylinder is of importance ; if this 
is bent at right angles, soot accumulates on the angle, and 
w r hen a mass has formed it falls off, and is partly burned 
in the flame. If the cylinder is given a proper bend no 
soot is deposited in it, but it is all carried away into the soot 
chambers. The air regulator, S, is placed at the bottom of 
the cylinder ; it must turn easily. The larger the slits are 
made by the rotation of this regulator, the more oxygen 
enters the flame, and the more vigorous is the combustion. 
A small, well-fitting door is placed in the lower part of the 
cylinder, through which the wick can be reached ; opposite 
to this a glass plate is inserted, so that the flame may be 
observed without opening the door. The wick is raised or 
low r ered by means of the screw, R. 

The oil reservoir, 0, must be outside the cylinder : in the 
older arrangements it is so placed that the wick sucks up the 
oil ; the workman who is in charge of the lamps must then 
take particular care that the reservoirs always contain the 
proper quantity of oil, if he neglects to keep the oil at the 
proper level the wick is charred ; it then sucks up too much oil, 
which cannot completely burn and is chiefly distilled, so that 
the soot is oily and cannot easily be treated afterwards. The 
most diligent and . attentive workman who is in charge of 
a large number of lamps may easily allow one of them to 
run short of oil. With the oil reservoir above depicted, a 
new quantity of oil flows only when the level of the liquid 



304 MINEBAL AND LAKE PIGMENTS. 

sinks below the line, U. As soon as a small quantity is nsed 
air enters the reservoir, 0, in place of which oil runs out 
until the opening, U, is again closed by the liquid. This con- 
struction of lamp only works well when thin oils are burned, 
and great care must be taken that the lamps are kept con- 
stantly clean. 

Lamps of all constructions have some drawbacks, they 
must always be cleaned, and losses of oil occur in filling. These 
defects are obviated when, instead of providing each lamp 
with its own reservoir, a single one is used for a large 
number of lamps, which are automatically fed from it. In 
this case the attendant has only to regulate the air supply 
to the lamps, and to see that the mechanical arrangement 
by which the oil flows to the different lamps is working 
properly. When the lamps are automatically fed, the burners 
must be firmly fixed, and all in the same horizontal plane. 
A pipe connects each burner with a main pipe running under 
the lamps, which latter pipe is connected with the reservoir, 
and this in its turn is connected with another reservoir 
placed a little higher. In the , pipe connecting the two 
reservoirs is a tap, which is opened by a float in the lower 
vessel as soon as the level of the liquid in it sinks a little, 
and which remains open until the level of the liquid has 
again risen to a certain height. The float in the reservoir 
connected with the lamps is arranged so that the level of the 
liquid is slightly higher than the burners. Under the slight 
pressure oil continually flows to the burners, and it is not 
difficult to regulate its flow so that all which reaches the 
burner is burned. 

In using a new oil it is not easy at first to regulate the 
flow so that absolutely all is burned without any dropping 
off the burner ; to prevent the loss of this portion, the lower 
end of the air regulator is conical, at the apex of the cone is 
a small tube and under this a vessel in which the unburnt oil 



MANUFACTURE OF SOOT PIGMENTS. 



305 



is caught. In Fig. 32, S is the air regulator, T the vessel to 
catch the unbarnt oil, L the pipe leading from each burner 
to the common pipe H, G- the vessel in which a float regulates 
the flow from a larger reservoir, so that the liquid always 
remains at the same level. When tar oils are used, or thin 
mineral oils, the pipes which convey the oil to the burners 
may be made tolerably narrow, but when viscous oils or fish 
oils are burned, narrow tubes would offer too great resistance 
to the flow, so that it is always advisable to use fairly wide 




FIG. 32. 

pipes. Viscous oils become considerably more fluid at higher 
temperatures, it is therefore well to place the reservoirs in 
the same room as the lamps ; in winter the temperature of 
this room is kept fairly high by the burning of the lamps, 
and thus the oils remain fluid. 

In recent years hydrocarbons of very low boiling point 
have been placed on the market at low prices ; these produce 
a black of very good quality. On account of the low boiling 

point of these very inflammable liquids, particular care is 

20 



306 MINEEAL AND LAKE PIGMENTS. 

necessary in burning them ; they are very fluid and retain 
their fluidity at a low temperature. The reservoirs for them 
should be placed for safety outside the lamp room, and 
should be closed by an air-tight lid, with only one small 
opening through which air can enter. When these low 
boiling liquids are used, particular care must be taken to 
regulate the flow to the lamp, or large quantities will evaporate 
without being burnt. 

The materials used in the manufacture of soot black differ 
considerably in chemical constitution. Rosin, animal and 
vegetable fats, distilled oils and volatile hydrocarbons are 
used. Each of these substances gives different decomposition 
products when heated, to which proper regard must be had, 
since the quality of the black is dependent on them. The 
more difficultly volatile are the products of distillation, the 
higher must be the temperature at which the combustion is 
conducted, otherwise the soot will contain a considerable 
quantity of these products of distillation. In consideration 
of the great variety of materials used for making soot black, 
it is impossible to say exactly in what manner each should be 
treated : this must be left to the practical experience of the 
manufacturer. 



CHAPTER XLI. 

MANUFACTURE OF LAMP BLACK. 

THE variety of soot black known as lamp black is the best ; 
it is used for making copper -plate inks and black coach 
paints. Its price is much higher than that of ordinary soot 
black ; for the best qualities, twenty times the price of an 
ordinary black is paid. The materials already mentioned are 
used for making lamp black, fish oils and rancid vegetable 
oils, and in recent times mineral oil and tar oils. In regard 
to vegetable oils it is to be observed that it is advisable to 
use very rancid oil, which gives a larger yield of soot ; 
experience has shown that a very rancid oil requires a larger 
quantity of oxygen to burn without a smoky flame than 
a sweet oil. This indicates that a portion of the carbon in 
the rancid oil is present in such a form that it requires 
a higher temperature to burn it than is required for the 
carbon in sweet oils. The use of rancid oils for making soot 
blacks has thus two advantages, it is much cheaper and 
produces a larger yield. The only disadvantage, and not 
a very important one, attending the use of rancid oil is that 
the free fatty acids in these oils strongly attack the metallic 
parts of the lamps. This is especially the case with copper 
and brass ; only such portions of the lamps as is absolutely 
necessary should be made of these metals ; all other parts, 
and in particular the oil reservoirs, should be made of tin 
plate. 

Since the development of the coal-tar industry oils 



308 MINERAL AND LAKE PIGMENTS. 

distilled from coal tar have come into commerce at low prices. 
These oils consist of carbon and hydrogen and are equally 
volatile with the essential oils, to which class turpentine 
belongs. There are two kinds of these tar oils, light and 
heavy ; they differ both in specific gravity and in the boiling 
temperature, which ranges between tolerably wide limits. 
When the burning qualities of these oils are examined, 
they show a great difference in the quantities of oxygen 
necessary for complete combustion with a white flame with- 
out smoke. The more oxygen an oil requires to burn with 
a non-smoky flame, the more suited it is for the manu- 
facture of blacks. Generally speaking, the difficulty of 
completely burning these oils is in proportion to their specific 
gravity and the height of their boiling point. 

By the distillation of rosin, oils are obtained which also 
consist of carbon and hydrogen and form a useful material for 
the manufacture of soot blacks. The mineral known as 
earth-wax or ozokerite, a substance intermediate between 
asphaltum and petroleum, can also be used, but since it is 
a solid it must be burnt in troughs. 

In burning light oils the arrangements can be very simply 
made ; wicks are not required and thus a considerable outlay 
is spared. In place of the burner, shallow dishes are used 
into which the oil enters from below, replacing that burnt 
away. It is necessary to cool these dishes continuously from 
below, otherwise they would soon become so hot that the 
greater part of the oil would evaporate without burning, and 
it would also be very difficult to regulate the flame properly. 

Black can be obtained from very resiniferous coal by 
burning in furnaces with a regulated air supply. The pro- 
duct is, however, generally largely contaminated by ash, and 
can only be used for common purposes. Very resiniferous 
lignite gives a better product than coal ; the black can be 
partially separated from the ashes by shaking upon water 



MANUFACTURE OF LAMP BLACK. 309 

and then stirring, when the particles of ash sink to the 
bottom and the light black floats on the top. 

However carefully the manufacture of soot black is con- 
ducted, it will always contain, in addition to carbon, varying 
quantities of the products of distillation, partly solid and 
partly liquid. In consequence of these admixtures the soot 
will not be pure black, but will show a more or less brown 
tinge, which is clearly observed when the black is smeared 
on white paper. At a certain thickness of the layer of soot 
it will be seen that the colour is not black, but an impure 
brown. When such soot, as it is taken from the chambers, 
is chemically examined, it is found that it gives up a large 
quantity of soluble matter to different chemical reagents. 
By proper treatment it is possible to remove the admix- 
tures almost completely, so that nearly chemically pure car- 
bon remains. Such pure carbon can be made by boiling 
lamp black with strong caustic soda solution so long as the 
liquid is coloured, and when caustic soda can dissolve 
nothing more, the residue is boiled with aqua reyia until this 
no longer takes up soluble matter. The black is then w r ashed 
with water until free from every trace of acid, and the residue 
dried. By this treatment the soot is converted into a powder 
of the purest black hue which it is possible to obtain. It is 
now no longer soot, but chemically pure carbon in the non- 
crystalline form ; heated upon platinum foil it burns to pure 
carbonic acid without producing smoke or smell. In practice 
the purification of the soot is not carried to the extent of 
producing pure carbon ; this would be accompanied by a 
diminished yield of the pigment without increasing the 
commercial value of the product : the aim of the manufacturer 
is simply to produce a substance of a pure black appearance 
from the brown soot. To remove the brown substances 
present in the crude soot the solvent action of caustic soda 
solution can be used. The soot is several times boiled in 



310 MINEEAL AND LAKE PIGMENTS. 

iron pans with strong caustic soda solution in order to dis- 
solve the products of dry distillation. It is superfluous to 
repeat this operation until fresh caustic soda remains colour- 
less. The treatment may be stopped when the solution 
acquires a slight brownish colour. When the soot has been 
purified so far it has lost its brown shade and now appears 
as a velvety black powder, very soft and very light, and 
distinguished by extraordinary covering power. 

It is not so easy as it appears at first sight to judge of 
the quality of a black pigment by its appearance. To the 
unaccustomed eye a pigment may appear to be of an unexcep- 
tionable black, which to the expert appears decidedly brown ; 
only long practice can give the eye the requisite keenness. 
In addition to the test already given of smearing on white 
paper there is another especially to be recommended to the 
inexperienced for discriminating between pigments. A small 
quantity of the black under examination is intimately mixed 
with a white pigment ; white lead or zinc white is very 
suitable. If the mixture has a pure grey shade, the black 
may be regarded as of good quality, but if it contains brown 
substances the mixture has an indefinite, dirty shade instead 
of a pure grey. This is a sure sign that the black requires 
further purification. 

Although caustic soda is now very cheap, its use for the 
purification of soot black is tolerably costly, because it 
entails much labour. This process is therefore only used 
for the finest qualities which are to be employed for copper- 
plate inks and black coach colours. For inferior qualities the 
method of calcination is used, which produces, when properly 
carried out, a black of such purity that it can be used for the 
preparation of even the finest black paints. 

Calcination of the Soot. The substances which give the 
soot its brown colour are products of dry distillation, and 
hence are all volatile at a certain temperature ; they can be 
separated from the soot by heating it in the absence of air. 



MANUFACTURE OF LAMP BLACK. 311 

The temperature necessary completely to volatilise these 
compounds is fairly high. The soot must be heated to a 
good red heat to obtain a safe result. 

When the soot is heated too quickly or to too high a tem- 
perature it undergoes an alteration which affects the quality 
of the product. By too long or too vigorous ignition the soot 
changes its flocky consistency to a sandy nature ; it will then 
require much longer grinding with oil to produce a uniform 
mixture than is the case with the light flocculent soot, which 
very readily mixes with oil. 

The soot is ignited in sheet-iron boxes with a coating to 
protect the metal from burning. This coating is best made 
from clay and hair. A very thin paste of clay and water is 
painted uniformly over the boxes with a brush ; when the 
first coat is dry a second and third are given. When once the 
metal is completely covered with clay, several coats are given 
of clay mixed with chopped tow until the layer is several 
millimetres thick. The coating carefully made in this manner 
is very durable, and the boxes can be used for a long time, 
whilst without the coating they would very soon be burnt. 
Particular care must be given to the manufacture of the boxes 
themselves ; the bottoms must fit very accurately, and should 
be coated with clay in order to ensure an air-tight joint. The 
lids also must fit accurately, and when they are closed must 
equally be made tight by clay. 

The soot is at first loosely packed into the box and each 
portion then .pressed with a rammer so that it tightly fills 
the boxes. In the lid is a very small opening, through 
which the volatile products can escape. The heating begins 
quite gently at the back, and proceeds gradually to the 
front ; the boxes are finally brought to a good red heat, at 
which they are maintained for about half an hour ; at this 
temperature the substances mixed with the carbon are almost 
entirely volatilised, and the soot acquires its true black appear- 
ance. The soot itself attains a red heat in the boxes and in 



312 MINERAL AND LAKE PIGMENTS. 

consequence of its loose nature it very readily burns. The pre- 
cautions given above must be observed in order to protect 
completely the soot against the action of the air. The boxes 
should not have the smallest opening besides that in the 
cover necessary for the escape of the volatile matters ; through 
an opening invisible to the naked eye so much oxygen may 
enter during the cooling of the boxes that a considerable 
quantity of carbon will burn to carbonic acid. 

To avoid losses through the carbon burning precautions 
must equally be taken in the cooling of the boxes. When 
the ignition is finished they are drawn out of the furnace by 
tongs and placed upright on a stone floor ; as the cool air 
enters, and in contact with the red-hot carbon would burn a 
portion of it. This can be prevented by a simple artifice : a 
red-hot coal is placed on the small opening in the lid ; this 
converts the oxygen of the air which enters the boxes into 
carbonic acid. When all the boxes are taken out of the 
furnace the doors of the room in w T hich they are placed are 
opened, so that they are cooled in the draught. Finely- 
divided carbon takes fire at a temperature far below a red 
heat, so that the boxes should not be opened until their 
contents are quite cold. If they were opened whilst hot the 
soot might take fire. 

Pine Black. Under this name a poor quality of black 
comes into the market, which is much used for ordinary black 
paints, etc. The pine black formerly brought into the market 
was what its name purported ; it was made from the roots of 
the pine in the primitive fashion already described. The black 
was sold without further purification : it w T as a soft light 
powder, very variable in shade on account of the absence of 
control over the process : it varied from pure black to a dark 
brown. At present, under the name of pine black many 
substances are sold, generally without purification, made 
from the most different materials, frequently from rosin, rosin 
residues, and other cheap materials. 



CHAPTEK XLIL 

THE MANUFACTURE OF SOOT BLACK WITHOUT CHAMBERS. 

A CERTAIN temperature is necessary for the combustion of 
every substance ; if a cold body is placed in the flame of a 
candle or an oil lamp it becomes covered with soot, because it 
cools the flame to such an extent that the carbon is no longer 




FIG. 33. 

heated to the temperature necessary for its combustion, and 
is, therefore, separated in a finely-divided condition. Use is 
made of this phenomenon in a method of making soot black 
which has many advantages, of which the most important is 
that expensive buildings are not required, and the manufacture 
can be conducted in restricted space. 



314 MINEKAL AND LAKE PIGMENTS. 

A convenient arrangement for obtaining black by this 
process is represented in Fig. 33. The thin- walled hollow 
cast-iron cylinder is turned smooth on the outer surface, and 
is surrounded by a sheet-iron cover at the distance of a few 
centimetres ; it rotates in bearings, which, like the spindles, 
are hollow ; cold water is thus led through the cylinder from 
a tank at a higher level. Below the cylinder are placed, 
near to one another, smoking lamps, and at the side of the 
cylinder is a broad brush of soft hair which continuously 
removes the soot deposited on the surface, which then falls 




FIG. 34. 

over an inclined plate into the collecting vessel. The 
cylinder is kept in slow rotation by any mechanical arrange- 
ment. The lamps are devised not to produce a hot flame, 
but when this comes in contact with the cold surface of the 
cylinder, it deposits a ring of soot on the rotating cylinder 
which is then removed by the brush. The cylinder is kept 
cooled by the water which runs through it. The soot which 
is collected shows a tolerably strong brown shade in con- 
sequence of the rapid cooling of the flame, which causes the 
formation of a large quantity of products of distillation ; the 
soot will always require ignition. 



MANUFACTURE OF SOOT BLACK WITHOUT CHAMBERS. 315 

It has been proposed further to simplify the manufacture 
of soot black by this method, by burning coal gas under the 
cylinder from numerous small openings in a pipe. Coal gas 
indeed gives a very fine deep black, but the yield is so small 
that this process could never be adopted with advantage. 

The apparatus constructed by Thalwitzer for the manu- 
facture of oil black (Fig. 34) consists of a plate, A, with a 
rim, a, fastened to a vertical axis, b. This is carried by 
bearings in B, and is kept in rotation by the cog-wheels, d 
and /, moved from the shafting, 0. The plate is cooled by 
water supplied by the pipe, g, it flows away by h and the 
annular vessel, D. The lamps, e, are connected with the 
common oil reservoir by n. H is a scraper fastened to B 
by x. 



CHAPTER XLIII. 

INDIAN INK. 

INDIAN ink consists of purified soot mixed with gum, as 
binding medium, and a little camphor and musk. It is 
generally believed that Indian ink is made in China by a 
process which is still a secret ; this opinion is supported by 
the fact that the ink made in Europe is almost always 
inferior to the Chinese. It is quite possible that a substance 
which produces a particularly fine black is used in China to 
prepare the soot used for the ink, and which we do not yet 
know or do not use on account of its high price. But it 
appears that the difficulty of producing an ink equal in 
quality to the Chinese lies less in the quality of the soot 
for by the methods of purification already given we can 
make almost chemically pure carbon than in the extremely 
careful mechanical treatment of the soot with the other 
constituents of the ink. 

The genuine Chinese ink has a peculiar smell, which, 
when not hidden by musk, distinctly recalls the smell of 
burning camphor ; the camphor tree is a native of China, 
so that it is not impossible that the soot of the wood of this 
tree is used for the ink. The soot from camphor alone 
would be too expensive, so that it is probable that camphor 
wood is used from which camphor has been prepared, or 
that camphor soot is mixed with the soot of a cheaper sub- 
stance. In fact several varieties of Chinese ink show dis- 
tinctly the smell peculiar to soot produced by fat. The 



INDIAN INK. 317 

binding material is animal glue, which is most thoroughly 
ground with the carbon ; the secret of making good Indian 
ink lies in the careful grinding with the binding medium. 

According to Piou, who lived for a long time in China 
and learned the process there used, the black employed for 
the ink is made from the resins of the pine and other trees. 
The soot is sieved through silk, boiled with glue and water, 
and kneaded by hand with a little oil until the mass has 
become completely homogeneous. It is then left for some 
days, heated, and pressed into the moulds. This description 
is not very satisfactory ; the products obtained are far inferior 
to those made by the ordinary European process. 

The viscous mass obtained by long treatment of the 
black with the binding medium is slowly dried, and again 
ground when it has become thick. When a completely 
homogeneous paste has been obtained it is made into sticks 
which are pressed into moulds, and then very slowly dried 
so that they do not crack. Small cracks always occur ; 
they are filled in by a brush dipped in the thick paste, 
finally the sticks are entirely or partially covered with gold 
leaf. 

In order to impart to the ink the odour characteristic 
of the Chinese ink, it is sufficient to use good lamp black, 
made odourless by extraction with caustic soda, and to add 
a little solution of camphor in turpentine during the grind- 
ing. If it is desired to impart the musk smell which several 
Chinese varieties possess, a small quantity of a spirit extract 
of genuine musk may be added. 

The size solution is made by long boiling isinglass with 
water ; it must be so concentrated that it forms a strong jelly 
on cooling. A small quantity of acetic acid is added, so that 
the grinding may not be hindered by the viscous nature of 
the size, which the acid prevents from gelatinising. When 
the mixture has been made by long grinding, the mortar is 



318 MINEEAL AND LAKE PIGMENTS. 

warmed to 40 or 50. C., whereupon the acetic acid is soon 
volatilised, and the mass rapidly becomes very thick. 

Neutral Tint Black. A mixture of Indian ink, Chinese 
blue, and a very small quantity of madder lake forms the 
colour known as "neutral tint," the shade of which is a 
peculiar greyish violet. By alterations in the proportions 
of the constituents different shades of "neutral tint" are 
produced. 

APPENDIX. BLACK MINERAL PIGMENTS. 

Chrome Copper Black. When copper chromate is strongly 
ignited in air and then treated with boiling nitric acid a 
glittering black compound is obtained, which shows up well 
when printed on fabrics with albumin. This and all similar 
pigments are distinguished by great durability. 

Chrome Black. When a mixture of chromium oxide 
with varying quantities of ferric oxide is strongly heated, 
pigments are obtained of all shades from dirty yellow and 
green to the deepest black. This pigment is largely used 
in porcelain painting to produce a black, which also can 
be obtained by other but much more costly methods. The 
best mixture to produce a deep black contains one part of 
chromium oxide to four parts of ferric oxide. 



CHAPTEE XLIV. 

ENAMEL COLOURS. 

AN enamel is a glass distinguished from ordinary glazes by a 
much lower melting point, and generally by opacity. Most 
enamels are coloured by additions of metallic oxides. 

Without going in detail into the art of enamelling, some 
observations may here be given which appear indispensable 
for an understanding of the manufacture of enamel colours. 
The enamels are glasses, but not every glass can be used for 
an enamel, since the constituents of the glass react with the 
substances by which the enamel is coloured, and with certain 
colouring matters quite different shades to the intended 
might be produced. Of equal importance with the com- 
position of the glass used in the preparation of the enamel 
is the temperature at which the glass melts. Many colours 
bear but a low heat ; they decompose at a somewhat higher 
temperature and produce quite the wrong colour. This is 
especially the case with the enamels coloured purple-red by 
gold. 

The enamel colours are fixed by mixing them with an 
easily fusible glass (the flux), applying them to the object to 
be enamelled, whether metal, glass, or porcelain, and heating 
the painted article in muffles until the glass melts, and either 
dissolves the colouring matter or encloses it unaltered. 
According as the enamel melts as a whole, or the flux 
alone melts and encloses the colouring matter, so transparent 
or opaque enamels are produced. The former are really 



320 MINERAL AND LAKE PIGMENTS. 

coloured glasses, the latter are glasses in which is enclosed 
the sintered colouring substance. 

Whilst enamels were formerly of importance only for 
artistic purposes, they have recently attained great industrial 
employment ; not only is earthenware now covered with 
enamel, and thus made to resist the attack of chemical 
reagents, but boiler tubes are lined with enamel to prevent 
the formation of scale. 

The colours used for enamels must in all cases be metallic 
oxides ; for yellow, silver and antimony oxides ; for red, gold, 
copper oxide and ferric oxide ; for blue, cobalt compounds ; 
for green, copper oxide or chromium oxide. Two operations 
are required in the production of enamel colours, the pre- 
paration of the flux and its fusion with the real colouring 
substance. In most cases the latter operation takes place 
simultaneously with the fixation on the enamelled article, 
but the enamels may be melted and cast into lumps, which 
are then powdered. 

White Enamels, These are always ordinary crystal glass, 
to which tin dioxide or potassium antimoniate has been 
added. Particular care must be taken that only pure raw 
materials are used, for almost unweighable quantities of iron 
compounds are sufficient to impart to the enamel a yellow 
tinge. In almost all cases the glass must be decolourised by 
the addition of pure pyrolusite (manganese dioxide), which 
at a red heat gives up a portion of its oxygen to the ferrous 
oxide contained in the glass, producing ferric oxide, w r hich 
has much less colouring power. Too large a quantity of 
pyrolusite imparts a blue tinge to the enamel. 

The tin dioxide used in the glaze is made by direct 
oxidation of tin in air. It has been found that tin burns 
much more easily in air when it is mixed with lead ; 20 to 40 
parts of tin are melted with 100 parts of lead, and the alloy 
heated in shallow vessels in the air ; it takes fire at a certain 



ENAMEL COLOUES. 321 

temperature, and it is only necessary to remove continually 
the layer of oxide in order to oxidise the whole of the metal 
within a short space of time. The mixture of oxides ob- 
tained in this manner is freed from particles of metal by 
grinding and levigation ; when it is fused with the glass, the 
lead oxide enters into combination, whilst the tin oxide is 
embedded in the colourless glass. When paste enamels are 
required the mass is melted in shallow crucibles, poured into 
water and broken up to a coarse powder, which is again 
fused. In some cases this operation must be repeated several 
times to obtain a quite homogeneous product, for tin dioxide- 
is very heavy and sinks to the bottom of the fluid glass ; this 
is readily seen if the fused enamel is allowed to cool in the 
crucible., When the crucible is broken the lower portions 
of the fused mass are dense white, whilst the upper are 
merely milky white. By repeatedly fusing the mass it is 
endeavoured to obtain it uniform. In fusing the enamel 
care must be taken that it is heated in a vessel from which 
the fire gases are completely excluded, since the smallest 
quantity of coal or of ferruginous ashes coming into contact 
with the melted mass would injure its colour. If the melting 
point of the enamel is too high it can be lowered by thej 
addition of a small quantity of pure quartz sand. 

A fine white enamel is obtained by using litharge or red 
lead. The mixture contains 60 parts of quartz sand, 30 of 
alum, 35 of common salt and 100 of red lead. It is advisable 
to add a small quantity of finely-powdered talc to the sand. 
In consequence of the considerable proportion of alumina con- 
tained in this enamel it is difficult to melt, and can be heated 
to very high temperatures without injury to the shade. 

When antimony oxide is used a glass free from lead must 
be taken ; lead glass does not give a pure white with antimony 
oxide. A very good white enamel is obtained by fusing 3 

parts of crystal glass with 1 part of sodium antimoniate. 

21 



322 MINEEAL AND LAKE PIGMENTS. 

Coloured Enamels owe their colour to metallic oxides. 
Enamels coloured by metallic oxides, in which the oxygen is 
firmly united, can be fired without great precaution, but if 
they contain oxides easily decomposed, great care is required 
to obtain a fine colour. In enamelling metals a white under- 
glaze is generally used beneath the coloured enamel ; it con- 
sists of a refractory white enamel. By this means it is more 
easy to obtain pure colours. 

Yellow Enamels are coloured by silver, antimony oxide 
combined with litharge, or ferric oxide ; from the latter a red 
can also be obtained. To produce a yellow enamel by means 
of silver the article is first enamelled white at a low tempera- 
ture ; silver oxide is then applied where required, and the 
article again heated. It then frequently happens that the 
surface has a metallic appearance owing to the reduction of a 
part of the silver oxide to metallic silver. When this coating 
is scraped off the enamel beneath is found to be coloured a 
fine yellow. 

The antimony yellow is obtained by mixing 1 part of anti- 
mony oxide, 1 part of alum and 1 to 3 parts of white lead, 
according to the depth of colour required. These finely- 
powdered materials are intimately mixed with 1 part of sal 
ammoniac, and heated in an open vessel with stirring until 
the yellow colour appears. The vapours of the ammonium 
chloride indicate the proper temperature for the operation. 
When this substance is completely volatilised the temperature 
should not be further raised, or the mixture would fuse. 

By means of ferric oxide a fine and durable yellow is pro- 
duced ; the quantity employed must not be too large, or a 
red colour will result. A very high temperature is required 
for burning in this colour. The alum used in making the 
yellow enamel colours serves to prevent the oxides from 
fusing. 

Red Enamel. Ferric oxide is generally used. When a 



ENAMEL COLOURS. 323 

purple red is required it is obtained either directly from 
metallic gold or from purple of Cassius. 

Eed iron enamel is made by slowly heating 20 to 25 parts 
of pure ferrous sulphate with 10 parts of aluminium sulphate 
until all the water of crystallisation is expelled, when the 
temperature is gradually raised to an intense heat. The 
shade depends upon the temperature : the higher it is the 
darker is the colour. Tests are taken from time to time of 
the mixture, and rapidly cooled. Hot ferric oxide is black, 
consequently the shade can only be judged with complete 
certainty in a cooled portion. Although the temperature re- 
quired for the preparation of this colour is very high, yet it 
ought not to rise so far that the magnetic oxide is produced, 
a small quantity of which would cause the enamel to appear 
dirty red, since it gives to the flux a blackish green colour. 
The red ferric oxide colour should not be dissolved in the glass 
of the enamel, but should be embedded in it. Thus, when 
the enamel is burnt on, the temperature should not rise so high 
that ferric silicate is formed, otherwise a yellow or even com- 
pletely black enamel results. 

Purple red is obtained by means of gold ; formerly purple 
of Cassius was exclusively employed for this purpose, but an 
equally bright red can be obtained from gold chloride. The 
colour obtained from gold will bear only a very low heat ; it 
must be mixed with a very fusible glass, brought on to the 
article to be enamelled, and heated just sufficiently to melt 
the mass. The following mixture is used for gold red and 
other delicate colours : 3 parts of calcined borax, 3 parts of 
quartz sand and 1 part of chalk. A very small quantity of 
gold is sufficient to produce a deep red, the amount of gold 
preparation used for a pale red or a medium purple must be 
carefully weighed. 

Blue Enamels are always coloured by cobalt oxide. Any 
cobalt compound can be used, since at a red heat the silica of 



324 MINERAL AND LAKE PIGMENTS. 

the glass displaces other acids, and produces cobalt silicate. 
It is most simple to take cobalt nitrate. This salt is readily 
obtained completely pure, and absolute purity is of great 
importance, for only thus is a pure blue produced. 

Cobalt oxide produces a more or less deep blue when 
fused with varying quantities of glass. In order to produce 
shades similar to turquoise and forget-me-not, a white enamel 
must be used beneath the blue, or bone-ash must be mixed 
with the blue enamel ; this produces a paler colour, which 
is also opaque. 

Green Enamel is coloured by copper oxide or chromium 
oxide. In the former case 1 part of copper oxide is used to 
30 to 50 parts of glass, according to the depth of shade required. 
The enamel made with copper oxide alone has never a pure 
green colour ; it exhibits a blue tinge. A pure green is ob- 
tained by adding a very small amount of ferric oxide, which 
produces a yellow and compensates for the blue shade, so 
that a pure green is formed. 

Chromium oxide produces a beautiful emerald green with- 
out any addition. The enamel may be exposed to very high 
temperatures without injury. 

Green enamels may also be made by mixing blue cobalt 
enamel with a yellow enamel ; an unexceptional shade results, 
but this method is seldom used, since green enamels are 
obtained in a simpler manner from copper and chromium 
oxides. 

Violet Enamel, Manganese dioxide is used exclusively to 
produce violet enamel ; an extremely small quantity is suffi- 
cient to colour a considerable quantity of glass. A pure violet 
shade is only obtained when very pure manganese dioxide is 
used : the artificially prepared substance should be used to 
produce the finest violet : the cost is considerably greater than 
that of the mineral pyrolusite, but equally good results can 
rarely be obtained from pyrolusite. 



ENAMEL COLOURS. 325 

Black Enamel, When a large quantity of ferric oxide, 
copper oxide or cobalt oxide is fused with a glass, a deep 
black enamel is obtained. Generally mixtures of these oxides 
are used ; experience ha.s shown that a much deeper black 
is thus obtained than from any one alone. 

Now that enamels are not used exclusively for artistic 
purposes, but in considerable quantities for earthenware and 
other technical purposes, the manufacture of enamel colours 
has attained considerable importance ; works already exist 
occupied almost exclusively with this special branch of manu- 
facture. 

By Lacroix' process enamel colours are made which can 
be applied to porcelain without further admixture with a 
flux ; for example, a very fine blue is made by dissolving 300 
parts of pure alumina and 100 parts of cobalt carbonate in 
nitric acid, evaporating the solution to dryness, igniting the 
residue and fusing it with 300 parts of quartz sand free from 
iron, 900 parts of crystallised boric acid and 1,800 parts of 
red lead. A blue glass is obtained which easily melts, but 
is difficult to powder ; but if poured when fluid in a thin 
stream into cold water, it forms thin threads, which are very 
brittle in consequence of the rapid cooling, and may be easily 
converted into a soft powder. 



CHAPTEE XLV. 

METALLIC PIGMENTS. 

METALLIC pigments do not always consist of metals ; the 
name is also applied to compounds which possess a pro- 
nounced metallic lustre ; mosaic gold, the preparation of 
which has been previously described, is an example of such 
a pigment. Another variety of metallic pigment is made by 
heating finely-powdered alloys : a layer of oxide is thus pro- 
duced upon the surface of the metallic particles, this layer 
produces a shade of the colour of the metal or alloy. Metallic 
pigments are only unalterable when they are composed of 
metals which are not changed by the action of the air. As 
a matter of fact, this property is not possessed by any of 
the metals used for this purpose ; even gold and silver are 
blackened by the action of the sulphuretted hydrogen in the 
air. This alteration proceeds, however, very slowly when the 
metal is enveloped by a layer of a binding medium, which is 
always the case when the metallic powder is used for paint- 
ing. There are many manuscripts, the initial letters of which 
have been coloured by gold or silver, in which the metals 
retain their peculiar lustre after the lapse of centuries. 

Metallic paints made from yellow y alloys have never much 
durability : they always contain copper, which readily alters ; 
articles painted with imitation gold soon lose their lustre, and 
in the course of time become green. 

- Shell-Gold. This very expensive artists' paint is made by 
rubbing gold-beaters' refuse with gum solution upon a stone 
slab until a completely homogeneous mixture results, which is 



METALLIC PIGMENTS. 327 

then rapidly thickened over the fire and generally allowed to 
dry in small shells. The preparation so made is known as 
shell-gold : it is sold at very high prices. 

The operation of grinding the gold is very lengthy ; it 
considerably increases the cost of the already expensive 
material. The process may be considerably shortened if 
the gold is obtained in a very finely divided state by a 
chemical operation. For this process coins or broken 
jewellery is heated in hydrochloric acid, and nitric acid 
gradually added. The gold dissolves in the mixture. If it 
was alloyed with silver a white precipitate of silver chloride 
is formed, which is filtered off after largely diluting with 
distilled water. The solution is boiled for some time to 
remove excess of nitric acid. A solution of ferrous sulphate 
is then added ; the liquid at once becomes bluish black, and 
in a short time deposits a brown precipitate composed of 
chemically pure gold, which is so finely divided that it is 
almost without the characteristic glitter of gold. The pre- 
cipitate is filtered off, dried and preserved in stoppered 
bottles. In order to prepare shell-gold from this gold powder 
it is simply necessary to grind it with thick gum solution 
in a porcelain mortar ; under the pressure of the pestle the 
gold rapidly acquires its natural glitter. The grinding need 
only be continued until the gold and gum solution are. uni- 
formly mixed. The mixture so made should be at once 
filled into the shells, since on standing the gold would soon 
separate from the gum solution in consequence of its high 
specific gravity. _ 

Shell-Silver. Genuine shell-silver can be made by rub- 
bing silver leaf with gum solution upon a slab exactly as 
genuine shell-gold. In this case also the labour may be 
considerably lessened by converting the silver into a state 
of fine division by a chemical process. Silver is dissolved 
in nitric acid, which must be quite free from hydrochloric 
acid, or insoluble silver chloride would be formed. In the 



328 MINERAL AND LAKE PIGMENTS. 

operation brown suffocating fumes are evolved which attack 
the respiratory organs ; it should therefore be conducted in 
the open air or under a flue with a good draught. The silver 
solution, which is generally coloured blue by admixed copper, 
is diluted with a large quantity of distilled water. A sheet 
of copper is then dipped into the liquid and rapidly moved 
about in it. The silver then separates as a dark grey powder ; 
after washing it is chemically pure, and when ground with 
gum solution it produces genuine shell-silver. 

Shell-silver and shell-gold are rarely used in painting on 
account of their cost ; they are chiefly used for illuminated 
manuscripts. When these metallic pigments are to be used 
in oil painting, in place of gum they must be ground with 
a liquid which mixes with boiled oil or essential oils. For 
this purpose copaiba balsam is to be recommended. 

Imitation Silver. The imitation silver pigments are made 
from an alloy of tin and bismuth, or from a tin amalgam. 
The latter is most easily made by melting tin in a porcelain 
dish and adding one quarter of its weight of mercury ; the 
mixture is then well stirred, and allowed to cool. It quickly 
solidifies to a crystalline mass, which is tolerably brittle and 
can be powdered without difficulty. To obtain a substance 
of a true silvery appearance the amalgam must be converted 
to a powder of a certain degree of fineness. If the powdering 
is carried too far the product loses a great part of its metallic 
lustre, and acquires a dull grey colour. 

The bismuth alloy is made by fusing 100 parts of tin, 
adding 100 parts of bismuth, and then 10 parts of mercury. 
It is not absolutely necessary to add mercury, but the addi- 
tion has the advantage that the solid alloy is far more easily 
powdered. The imitation silver made by this process has 
a white metallic colour approaching that of genuine silver, 
but not equal to it, especially in lustre. It is, however, 
largely used in the industries on account of its low price ; 
for example, for paper hangings. 



CHAPTEE XLVI. 

BRONZE PIGMENTS. 

IT would be anticipated from the name that bronze pigments 
were composed of an alloy of copper and tin ; in reality the 
alloy is composed of copper and zinc, i.e., brass. The bronze 
pigments are made by a similar process to that described for 
genuine gold and silver pigments. The waste produced in 
the manufacture of imitation gold leaf is ground with a 
.solution of dextrine upon a slab until the mixture is uniform 
and separate metallic particles can be perceived only through 
a lens. Whilst genuine gold and silver paints are always 
made in small quantities, on account of the expensive nature 
of the material, and machinery is not employed, the bronze 
pigments are in different case ; the use of mechanical arrange- 
ments is necessary for producing the fine subdivision, other- 
wise the bronze would be very dear on account of the great 
cost of grinding. The mechanical arrangements required to 
divide the alloy are of the ordinary nature, but special 
machines have been constructed for the manufacture of 
bronze pigments, by which the metal is far more quickly 
converted into powder than by means of grinding machinery. 
These machines consist of metal drums studded on the interior 
with a large number of fine needles and capable of very rapid 
rotation. When a metal powder already tolerably fine is 
brought into these drums, it is rapidly brought to such a 
condition of fine division as would be attained by hand 
grinding only by prolonged and laborious exertion. 



330 MINERAL AND LAKE PIGMENTS. 

The raw material for the manufacture of bronze powders 
is produced in making imitation gold and silver leaf ; the 
waste metal obtained in beating the sheets is used. The 
employment of this waste has two advantages : the metal is 
already in very thin sheets and is composed of alloys varying 
in colour from silver white, through gold, to a bright copper 
In making leaf metal the waste of each colour is kept care- 
fully separate, so that it simply requires to be broken up to 
produce bronze powders of different shades. 

Before this waste is brought into the drums mentioned 
above it must be subjected to a preliminary grinding in a 
mortar with a small quantity of a fatty oil, which serves to 
bind together the mass. Sufficient oil should be used to give 
the mass some degree of coherence ; if too much oil is added 
the space between the needles of the drum would be coated 
with the mass and the process in the drum would require a 
much longer time. The uniform mixture of oil and bronze 
waste is then brought upon a wire sieve of the finest possible 
mesh, and the mass is rubbed through by means of a fine metal 
brush into a vessel below ; thus the larger particles are retained 
by the sieve and only those which are smaller than the mesh 
pass through. The product of this process is then brought into 
the drums, which are rapidly revolved ; the small particles of 
metal are thrown with great force against the side and are 
converted by the fine points with which it is studded into a 
very fine powder. The time required for this process de- 
pends on the rate of revolution and on the quantity of powder 
treated at once. The drums are stopped from time to time 
and the contents examined. When the powder is sufficiently 
fine it is taken out of the drum. This is most easily accom- 
plished if the drum is arranged to take apart into two halves. 

In most works it is usual to free the bronze powder from 
the admixed oil by subjecting the mass to the greatest 
pressure that can be produced by a very powerful hydraulic 



BKONZE PIGMENTS. 



331 



press. The oil which flows from the press is always green,, 
which shows that chemical action has taken place. In con- 
sequence of the large surface imparted to the oil it becomes 
speedily rancid, and then contains free fatty acids which 
attack copper very energetically. 

The use of hydraulic presses may be avoided by removing 
the oil by means of a solvent. Fatty oils dissolve very 
readily in carbon bisulphide, but this solvent cannot be 
used in this case, because commercial carbon bisulphide 
always contains dissolved sulphur, which would blacken the- 




FIG. 35. 

bronze powder. Petroleum ether and benzene are very suit- 
able solvents for this purpose. On account of the volatility 
of these inflammable liquids lights must be absolutely ex- 
cluded from the room in which they are used, and in order 
to avoid loss of solvent the bronze powder must be treated in 
closed vessels. The safest plan is to use a special apparatus 
of simple construction to dissolve the oil. Fig. 35 shows the 
construction of an arrangement suitable for this purpose. 
It consists of a cylindrical vessel of tin plate surrounded by a 



332 MINERAL AND LAKE PIGMENTS. 

rim into which fits the edge of the cover. When the lid 
is placed on and the rim filled with water, the contents 
of the vessel are closed in air-tight and cannot evaporate. 
The lower portion of the vessel is conical, and is joined to a 
pipe in which is a tap, and which communicates at the side 
by a tube with the glass vessel in which the solvent is con- 
tained ; another tube connects the neck of this vessel with 
the cover. When this apparatus is used for extracting the 
oil from bronze powder, a filter of strong blotting-paper is 
placed in the conical portion of the vessel, care being taken 
that it fits accurately so that it is not torn by the weight of 
the bronze. The oily bronze powder is placed on this filter, 
the cover set on, and the rim filled with water. By opening 
the tap attached to the solvent reservoir the liquid is allowed 
to enter the cylinder from below, the air in the latter passing 
through the tube in the cover to the reservoir. After some 
hours the oil is dissolved, the tap in the cover is then opened 
and the liquid run off by opening the lowest tap. If the bronze 
powder is not quite free from oil after one treatment with the 
solvent, the operation is repeated with a fresh quantity. 

When free from oil the powder and filter are removed 
from the apparatus and dried ; the dry mass forms a solid 
cake, which is broken up in a mortar and by a little grinding 
changed into a fine powder. 

The colour of the bronze powder is the same as that of 
the alloy used, but the shade is always rather paler than 
that of the coherent metal ; regard must be paid to this 
circumstance in making a bronze of a determined colour : the 
alloy employed must have a rather deeper colour than the 
shade the bronze is to possess. 

The manufacture of leaf metal and bronze powder is 
frequently conducted in the same works, which also often 
prepare the requisite alloys ; we, therefore, give a few examples 
of the composition of the alloys which produce certain shades. 






BEONZE PIGMENTS. 333 

The more zinc the alloys contain the lower is their melting 
point, the greater their brittleness and hardness and the paler 
their colour. An increase in the copper causes the colour of 
the alloy to approach more nearly to that of gold, and increases 
the malleability, a property useful in making leaf metal, but 
not desirable in making metallic powder. A zinc copper 
alloy which contains between 1 and 7 per cent, of zinc 
has an almost pure red, or even a dark red colour ; an alloy 
containing 7*4 to 13'8 per cent, of zinc has a pure golden 
yellow colour, between 16'6 and 25 per cent, of zinc a yellow 
appears. An increase of the percentage of zinc above this 
point produces the colour of brass ; it is noteworthy that an 
alloy containing still more zinc, 33 to 41 per cent., again 
shows a reddish colour, which is most developed when the 
alloy contains equal parts of zinc and copper. If the zinc is 
increased still further the shade gradually goes over to white ,- 
this change is already observed in an alloy containing 51 per 
cent, of zinc, which shows a pure golden yellow colour, and 
is very brittle. When the zinc rises to 53 per cent, the colour 
is reddish white ; at 56 per cent, it is yellowish white, at 64 
per cent, bluish white, and between 75 and 90 per cent, the 
alloy is bluish grey. 

The alloys for bronze pow^ders of different shades have the 
following composition, according to R. Wagner : 

Copper, per cent. 7Anc, per cent. 

Pale yellow 83 17 

Red 94-90 6-10 

Deep red 100 

Bronzes from English, French and Bavarian works con- 
tain the following percentages of copper : 

ENGLISH BRONZES. 

Orange 9'82 per cent. 

Deep yellow 82-37 

Pale yellow 80-42 



334 MINEEAL AND LAKE PIGMENTS. 

FRENCH BRONZES. 

Copper red 97'32 per cent. 

Orange 94-44 

Pale yellow 81-29 

BAVARIAN BRONZES. 

Copper red 98-92 per cent. 

Violet 98-82 . 

Orange 95-30 

Straw yellow 81-55 

Speiss yellow 82-34 

In each case the remainder of the alloy consists entirely 
of zinc. 

Alloys containing from 1 to 35 per cent, of zinc are only 
malleable in the cold. The malleability is at the greatest 
with a content of zinc between 15 and 20 per cent. ; such 
alloys are the most suitable for making leaf metal. Alloys 
containing between 36 and 40 per cent, of zinc may be ham- 
mered either cold or hot, whilst the former alloys become 
brittle on heating. When the percentage of zinc is still 
further increased the malleability decreases. The most brittle 
alloys contain 60 to 67 per cent, of zinc. 

The alloys are made in a furnace with a good draught, for 
copper liquefies at a very high temperature. To prevent loss 
of copper by oxidation the molten metal should not come in 
contact with air ; it should be covered by a layer of red-hot 
coal, which prevents oxygen from reaching it. When the 
copper is completely melted, which is ascertained by stirring 
with a piece of wood, the whole of the zinc is added. Some 
skill is required in this operation, otherwise a large proportion 
of the zinc will be volatilised, and the vapours will burn when 
they come in contact with air, in which case dazzling bluish 
white flames are seen over the crucible. The best method is 
to throw the zinc into the crucible and immediately stir it into 
the molten metal with a wooden rod. The products of the 
dry distillation of the wood, which are given off in great 



BKONZE PIGMENTS. 335 

quantity at this high temperature, keep the air from the 
surface of the metal and prevent the oxidation of the zinc 
vapours. The zinc is thoroughly mixed with the copper by 
stirring with the wooden rod, the crucible is then slowly 
cooled, with the precaution that the surface of the metal is 
kept covered by red-hot coals so long as the metal is fluid 
When sufficiently cool the metal is poured into shallow 
iron moulds, in which it quickly solidifies ; it is then rolled 
into sheets, which may be converted into thin leaves by 
hammering in a similar manner to that in which the gold- 
beater makes gold leaf. 

To obtain bronze powders of different shades alloys of 
different colours may be used ; the bronze powders may 
also be shaded by two methods either by adding certain 
colouring matters, of very great colouring power or by par- 
tially oxidising the finely divided metallic powder. In the 
first process the finely ground colouring matter is mechanic- 
ally mixed with the bronze powder. The use of manual 
labour would involve a great loss of time ; even when quite 
small quantities of bronze and colouring matter are mixed in 
a mortar it is necessary to grind diligently for a very long 
time before a mixture of homogeneous appearance is obtained. 
In working on a somewhat larger scale it is therefore advis- 
able to adopt mechanical mixing arrangements. A very 
simple apparatus suffices. A sheet-iron cylinder is used 
which can be revolved, and provided with a well-fitting slide. 
In this cylinder are placed the bronze powder and the colouring 
matter until it is about half full ; then, after tightly closing the 
slide, it is set in slow rotation, which is continued until a test 
taken out shows a uniform colour. 

When bronze powder is slowly heated in a shallow vessel 
the colour begins to darken at a temperature not much above 
the boiling point of water. In consequence of the fineness of 
the particles of the metallic powder the copper readily takes 



336 MINERAL AND LAKE PIGMENTS. 

up oxygen, and is superficially converted into copper oxide. 
This oxide is of a darker colour, and thus by this method 
the shade of the bronze can be deepened as desired. This 
simple operation requires a certain amount of practice to pro- 
duce a product of a determined shade. The desired result is 
most safely attained when the bronze is spread out quite 
uniformly in a thin layer upon a metal plate, which is gently 
heated from below. The powder soon begins to darken ; by 
cooling the plate the progress of the oxidation may be arrested 
at any moment. 

Kecently bronze powders have come into the market show- 
ing all possible colours in the deepest shades, by the aid of 
which very remarkable colour effects can be produced. These 
bronzes are made by dissolving an aniline dye in a little 
alcohol, pouring this solution over the powder, and mixing 
the dye uniformly through the whole of the bronze by work- 
ing the mass 'for a sufficient length of time. In this way 
bronze powders are produced which possess a green, red, blue 
or violet lustre, according to the colour of the dye used. 
These colours with metallic lustre can also be produced by 
bronzing the article with a white (zinc) bronze, and then 
coating it with a varnish in which the required aniline dye 
is dissolved. A bronze with a fine golden red glitter is pro- 
duced by applying a golden yellow bronze and then a varnish 
in which a little aniline red is dissolved. It should be ob- 
served here that these effects, produced by a coat of varnish 
in which an aniline dye is dissolved, only turn out well when 
the dye is used in very small quantity, for these colours are 
the strongest with which we are acquainted, and in colouring 
power far surpass cochineal carmine, which is renowned for 
this property. 

When bronze is coloured by dyes the most varied shades 
can be obtained with a metallic lustre. According to Conradty 
a very fine blue bronze is obtained by boiling white bronze 



BRONZE PIGMENTS. -337 

for some hours with a weak alum solution, - washing and 
drying, and then mixing lin a mortar with a strong solution 
of aniline blue in alcohol until the solvent has evaporated. 
This operation is repeated until the desired depth of shade is 
obtained. The bronze is then washed] with pure water. 
Conradty also recommends that the coloured bronze should 
be ground with a little petroleum, and then exposed to the 
air to allow the petroleum to evaporate. This operation, for 
which no chemical reason can be given, is quite unnecessary. 
If other dyes or mixtures of them are used in place of aniline 
blue, bronzes of corresponding colour are obtained. 

However handsome are the bronzes coloured by this 
process, nearly all have the disadvantage that the colours 
have little permanence, and quickly fade when exposed to 
light. This is especially the case when the bronzed article is 
coated with an oil varnish ; if, however, a spirit varnish is 
used, or indeed any varnish composed of a resin and a volatile 
solvent, the colour of the bronze, protected by the layer of 
resin, remains quite unaltered for a long time. 

Electrolytic Copper Bronze. Electrolytically precipitated 
copper may be used as a bronze pigment ; it is most simply 
made by adding pieces of metallic zinc to a solution of 
copper sulphate free from iron and violently shaking the 
flask for a long time. The liquid becomes warm, and the 
copper separates in the form of a very fine precipitate, which 
is collected on a filter and washed with air-free water (boiling 
water is best) and then quickly dried. The upper portions of 
the precipitate in the filter, which are exposed to the air, 
have generally a brownish colour due to the incipient oxida- 
tion of the finely divided metal. They are removed, and the 
lower portions show the characteristic colour of pure copper. 

In the same way silver can be precipitated from a solu- 
tion of silver nitrate in a finely divided state, but the 
particles of the silver powder are so very small that they 



338 MINEEAL AND LAKE PIGMENTS. 

reflect very little light, and consequently the powder has an 
unsightly grey colour. When a surface painted with this 
silver is rubbed gently with a hard body, the metallic lustre 
appears. 

Tungsten Bronze Pigments are expensive and rarely 
employed. They are obtained by fusing sodium or potas- 
sium tungstate in a porcelain crucible and gradually adding 
tungstic acid until the mass has an acid reaction. Tin 
dioxide is then added in quantity sufficient to neutralise 
the tungstic acid ; the mass is cooled and finely powdered. 
According as potassium or sodium tungstate is used, a violet 
or reddish pigment is obtained which exhibits the peculiar 
metallic lustre of a bronze powder. 

Still more costly is vanadium bronze, which is made by 
adding ammonium vanadate to a solution of 2 parts of 
copper sulphate and 1 part of ammonium chloride with con- 
tinual stirring, until the precipitate no longer re-dissolves on 
stirring. The liquid is then heated for several hours to about 
35 C., when vanadium bronze separates in golden yellow 
scales. These are collected on a filter, washed and dried 
When ground with oil or gum solution they can be used as a 
red gold bronze. The colour is unaltered by the air. 



CHAPTER XL VII. 

VEGETABLE BRONZE PIGMENTS. 

UNDER this apparently contradictory term, substances come 
into the market which produce a peculiar metallic lustre. 
When applied under certain conditions, the appearance is 
similar to that produced by real bronze. The vegetable 
bronze pigments are lakes as pure and free from foreign 
admixtures as possible. The lakes obtained from red wood 
or logwood can be used for this purpose. 

From the red woods (see p. 384) a magnificent bronze 
pigment can be obtained, which is either pure golden yellow 
or possesses a greenish golden metallic lustre not unlike the 
colour of the wing cases of the rose-bug. To obtain either 
shade a pure lake is first made by extracting red wood with 
boiling water, adding a little carbolic acid to the decoction 
(O'Ol per cent, of the quantity of liquid) and allowing to stand 
for several weeks. The liquid is syphoned off from the deposit, 
heated, and alum added equal in quantity to 10 per cent, of 
the wood used. The mixture is then allowed to stand for 
about a week, the precipitate is filtered off, washed, and, if 
necessary, dried. If the bronze is to be used in the form of 
water-colour, the precipitate is dried to a thick paste and 
mixed with about 10 per cent, of its volume of thick gum 
solution, so that a viscid mass is obtained which can just be 
applied with the brush. When the coating is made so thick 
as to hide the surface of the bronzed article, it has when dry 
the golden green colour. 



340 MINERAL AND LAKE PIGMENTS. 

In using this lake to prepare a pigment similar to gold 
bronze it must be almost completely dried, and then mixed 
with the liquid obtained in the following manner : White 
soap is melted on the water-bath with the smallest possible 
quantity of water, and when completely dissolved the same 
quantity of white wax is stirred in, finally water is added, so 
that the cooled liquid has the consistency of a moderately 
thick varnish. When this liquid is ground with the requisite 
quantity of the still damp lake and the mixture applied to 
paper, wood, or leather, and after drying rubbed with a glass 
ball, it gradually acquires a very fine go] den bronze colour. 
This method of bronzing is largely used in the manufacture 
of wall papers and for colouring fancy leather. To protect 
this coating against the action of water, it should be varnished 
when dry. 

These bronze pigments may also be used in varnish. The 
lake is then completely dried and ground with varnish in 
such quantity as to give a thick mass which can just be 
brushed on. 

The vegetable bronze is obtained from logwood in a 
similar manner, a solution of stannic chloride being generally 
used to precipitate the lake when a deep bronze is required, 
and alum for a pale gold shade. By using mixtures of the 
two salts intermediate shades are produced. The precipitate 
produced by alum may be shaded by means of potassium 
bichromate. Haematoxylin forms with chromium oxide a 
deep bluish black compound which has such intense colour- 
ing power that it is used to colour writing ink. The ink is 
prepared by adding a little potassium bichromate to a decoc- 
tion of logwood. If a very small quantity of this dark pre- 
cipitate is mixed with the lake precipitated from logwood 
extract by alum solution, colours are produced possessing 
the peculiar metallic lustre and a shade depending upon the 
quantity of potassium bichromate added. The addition of 



VEGETABLE BEONZE PIGMENTS. 341 

this salt must be very carefully made, as a very small excess 
is sufficient to render the colour so dark that it is useless as a 
Bronze. The shade of the precipitate depends upon the con- 
centration of the liquid and other conditions, so that it is 
impossible to give precise quantities. In practice, the safest 
and most convenient method is to dissolve the bichromate in 
a large quantity of water and add very small quantities of the 
dilute solution to the logwood extract along with the alum. 
After each addition a portion of the precipitate is rapidly 
mixed with the above-mentioned solution of soap and wax, 
to which a little size has been added, and then spread upon 
paper. If the desired shade has not yet appeared, a little 
more bichromate is added and another test made, and the 
process repeated until the proper shade is attained. 

Pigments for colouring wall papers and fancy leather are 
not easily made which produce such fine effects at so small 
a cost as the vegetable bronze pigments, which deserve the 
greatest regard from the colour maker and leather and paper 
manufacturers . 

Appendix The Brocade Pigments (" Brocatfarben "). 
Under this designation powders have been recently intro- 
duced characterised by a strong metallic or glassy lustre, and 
very suitable for certain purposes, such as the manufacture 
of wall papers, since they enable remarkably fine effects to be 
produced. They consist of mica in a tolerably fine state of 
division. Mica is a mineral which occurs frequently in nature 
and which very readily splits into thin sheets ; it occurs in 
various colours. In thin sheets mica is colourless and com- 
pletely transparent ; in somewhat thicker pieces it generally 
shows a distinct metallic lustre similar either to that of gold 
or silver. When^ground to fairly fine powder, it has the 
same gold or silver lustre, and gold or silver brocade colours 
are distinguished in commerce. They are made by grinding 
mica which has been previously sorted according to its ap- 



342 MINERAL AND LAKE PIGMENTS. 

pearance. The powdered mica can be sold as powder or may 
be mixed with a binding substance in order to be ready for 
use. Gum Arabic is generally used as the binding medium, 
but it can be replaced by the cheaper dextrine. When printed 
upon paper the brocade pigments produce the effect of a 
bronze, and in addition to their cheapness they have the 
great advantage of being completely unaltered by the air. 



PIGMENTS OF ORGANIC ORIGIN. 

CHAPTEK XL VIII. 

LAKES. 

THE pigments commercially designated lakes generally con- 
sist of an organic colouring matter united with a metallic 
oxide. In isolated cases other compounds of colouring 
matters are included under the term, such as indigo sul- 
phonic acid, and yet more rarely pure colouring matters are 
included, for example carthamine red. But the overwhelm- 
ing majority of lake pigments are compounds of a colouring 
matter with the oxide of a metal. For this purpose the 
oxides of tin, lead and aluminium are commonly used. 

Lakes are generally made by mixing the solution of the 
colouring matter with the solution of a salt of the metallic 
oxide, and precipitating the oxide by an alkali. The colour- 
ing matter is separated along with the oxide and forms 
with it a substance known as a lake. It has not yet been 
decided whether the lakes are true chemical compounds of 
the colouring matter with the metallic oxide, or whether 
the colouring matter is simply held fast by the surface 
attraction of the finely divided oxide. In favour of the 
latter view is the fact that a larger or smaller quantity 
of the colouring matter can be united with a given quantity 
of the oxide. 

The lakes vary greatly in durability ; some, such as the 



344 : MINEEAL AND LAKE PIGMENTS. 

madder lakes, can be counted among the most durable pig- 
ments, whilst others have very little permanence, as for 
example, the logwood lakes. 

The majority of the colouring matters used in the pre- 
paration of lakes are of vegetable origin, but several are 
derived from animal sources. The properties of the colour- 
ing matters used for this purpose vary greatly : each material 
demands a special treatment for the production of lakes. 
Therefore, in describing the methods by which lakes are 
made, we shall give first the properties of the colouring 
matter in question, and then proceed to the preparation of 
the lake.? - In the preceding portion of the book the pigments 
have been arranged according to their colour ; the lakes will 
be described in the same order. The yellow, red or green 
lakes of different origin will be treated in the same section in 
order to facilitate reference to any particular pigment. 

White lakes are not known, neither are there lakes which 
can be described as black. "With these exceptions lakes of all 
colours can be made and also of all shades. As an example 
of this madder lake may be given, which is found in commerce 
in a great variety of shades ; it can be made from the palest 
rose red to the deepest purple red, or, more properly, madder 
red, which is a characteristic shade. The shades of any lake 
are obtained by mixing in a white pigment just as in the 
manufacture of mineral pigments. Whilst, however, in the 
latter case the white pigment must often be mechanically 
mixed with the colour, the shading of the lakes is accom- 
plished in their preparation. Generally speaking, a pale 
shade of a lake is obtained by increasing the amount of the 
salt, the oxide of which is used to precipitate the colouring 
matter. The oxides in question are white (with the exception 
of lead oxide) ; thus in the pale shades a small amount of 
colouring matter is precipitated upon a large amount of oxide, 
whilst in the deep lakes the reverse is the case, a large quantity 



LAKES. 345 

of colouring matter is precipitated upon a small quantity of 
oxide, and the shade thus appears very deep. 

Apart from the nature of the colouring matter contained 
in a lake, and considering only the metallic oxide with which 
the colouring matter is united, it appears that lakes containing 
lead oxide have little durability. The combination between 
the colouring matter and the metallic oxide in a lake is so 
loose that it is easily destroyed by sulphuretted hydrogen ; if 
a lake containing lead is exposed to the action of air containing 
that gas the colour will, in the course of time, inevitably 
blacken. The lakes containing tin are also susceptible to 
sulphuretted hydrogen ; in air .containing but a trace bf this 
gas they quickly lose their brilliance and in time are quite 
discoloured. 

Alumina is not affected by sulphuretted hydrogen, and 
must thus be regarded as the most suitable oxide for the 
preparation of lakes. It is generally applied in the form of 
alum, in the selection of which great care is necessary if fine 
colours are to be produced. Commercial alum frequently 
contains ferric oxide ; when the alumina is precipitated simul- 
taneously with the colouring matter the ferric oxide is also 
thrown down, and is mixed with the lake, its dark colour 
influencing the shade of the lake greatly to its disadvantage. 
The effect of the ferric oxide upon the colour of the lake is so 
important that it is impossible, for example, to obtain a pale 
red lake with alum containing iron. To avoid the bad results 
given by alum containing iron it should be examined before 
it is used, and if it is found to contain any considerable 
quantity of iron it should be rejected for this purpose. 

After alum a solution of stannic chloride is most com- 
monly used to precipitate lakes, but it must be free from stan- 
nous chloride ; it produces as a rule darker lakes than alum. 
Care must be taken that the stannic chloride solution is free 
from iron. 



346 MINEBAL AND LAKE PIGMENTS. 

The ordinary process for making lakes is very simple : 
a clear aqueous solution of the colouring matter is obtained ; 
to this alum or stannic chloride solution is added in proportion 
to the amount of colouring matter dissolved by the water ; the 
metallic oxide is then precipitated by an alkali. Sodium or 
potassium carbonate or caustic alkalis may be used ; ammonia 
is very suitable for this purpose, since it is free from iron. 
The precipitant must be cautiously added, it is introduced in 
drops when the greater part of the colouring matter has been 
precipitated ; an excess of the alkaline solution would have 
considerable effect on the shade of the precipitate. The pre- 
cipitated lake rapidly settles to the bottom of the almost 
colourless liquid ; the settling is particularly rapid when 
stannic chloride has been used. The liquid is then drawn 
off, the pasty residue brought upon a strainer, washed several 
times with water, and dried in the air or in stoves. 

When dry, a properly prepared lake forms a mass of little 
coherence, and can be readily ground to a soft powder, which 
may then be ground with oil or gum solution to produce oil 
or water paints. At the same time the materials are added 
to the pure lake which are used to shade its colour ; white 
pigments are generally employed for this purpose the pale- 
ness of the shade is proportional to the amount of white 
pigment introduced. Certain lakes are shaded by small 
quantities of other colouring matters ; for example, the ad- 
mixture of a small quantity of a blue pigment with a pure 
red lake produces a much deeper colour inclining to purple. 
If a small quantity of a red or blue colouring matter is added 
to a yellow pigment there is produced a deeper yellow inclining 
to orange, or a yellow inclining to green, in proportion to the 
quantity of the added colouring matter. 

It should be observed that it is of great importance for 
the colour-maker to have a practised eye, sensitive to fine 
differences of colour. He may then produce colours faultless 



LAKES. 347 

in shade, for if by chance a colour does not turn out well 
it may be improved by judicious admixtures. It is quite 
impossible to give definite rules for the mixing of colours ; 
the estimation of a shade cannot be taught in words, it 
demands long practice. By the use of a system, provided 
the user has normal eyes, very slight differences of colour 
can be readily estimated. A scale of colours should be made 
from pigments of pure and definite shades, in which scale 
successful mixtures are gradually inserted, so that a series is 
produced in which the separate pure colours blend regularly 
into one another. Then, when it is required to produce a 
pigment corresponding to a certain shade on the scale, with 
a little practice the shade can be readily produced, for the 
scale indicates which colour predominates. 

The so-called sap-colours are generally characterised by 
very pure shades. They are produced by decomposing a 
lake by an acid or a strong base, evaporating the solution 
of the colouring matter so obtained at a gentle heat and 
mixing with gum, starch, or some other thickening material 
to produce a mass of such consistency that it can be formed 
into balls or sticks. 

The decoctions of the dye-woods always contain other 
substances in addition to the colouring matters, which are 
precipitated with the colouring matter, and somewhat injure 
its shade. When such a lake is decomposed the colouring 
matter is obtained in a purified form, thus showing its 
full beauty. This solution of the purified colouring matter 
might be again precipitated with a salt solution and lakes be 
produced which would be much brighter than the original, 
but the losses incidental to this double precipitation would 
be so great that the lake would be made much too expensive. 



CHAPTEK XLIX. 

YELLOW LAKES. 

YELLOW colouring matters are widespread in nature ; from 
them are obtained many yellow pigments used in dyeing and 
in the manufacture of colours. The colours produced by the 
yellow vegetable colouring matters are not particularly bright ; 
thus the yellow lakes are used chiefly to produce cheap 
pigments, whilst the inorganic pigments are used for bright 
yellows. 

Dutch Pink. -Several species of buckthorn (PJiamnus) 
contain a yellow colouring matter xanthorhamnin which 
is obtained pure by extracting the yellow berries with hot 
alcohol. On cooling, the impure colouring matter separ- 
ates ; by repeated recrystallisation from alcohol it is obtained 
in the form of crystalline needles, which are soluble in water 
and alcohol. 

The yellow lake known as Dutch pink is prepared from 
yellow (Persian) berries, by boiling the crushed berries with 
water and mixing the extract with a solution of alum. The 
lake is then precipitated by the addition of powdered chalk. 
As a rule, 500 parts of water are used to 100 parts of berries, 
20 parts of alum are added to the decoction, and the mixture 
poured upon 75 parts of finely powdered chalk. The liquid 
is decanted off, the residue filtered, washed and dried. Com- 
mercial Dutch pink is made from a mixture of the decoctions 
of yellow r berries, quercitron bark and turmeric, to which the 
alum solution is added, and then chalk. The precipitate is 






YELLOW LAKES. 349 

made into conical lumps, which are sold as Dutch pink, and 
used for ordinary painting and for colouring leather. 

Weld Lake. The dyers' weld (Reseda luteola) contains 
a yellow colouring matter formerly much used in dyeing ; 
a yellow lake can also be obtained from it. When weld is 
boiled with water a deep yellowish green decoction is obtained, 
from which yellowish green flocks separate on cooling. The 
lake can be obtained from all parts of the plant except the 
root", the flowering shoots giving the largest yield of colour ; 
the lake is generally precipitated by alumina. Equal parts of 
weld and alum are boiled with water until the latter is 
dissolved and the liquid is coloured deep yellow ; the hot 
solution is quickly filtered through a thick linen cloth and 
soda solution gradually added in small quantities with con- 
tinual stirring so long as effervescence follows. 

Alum entirely free from iron is required to produce 
a bright yellow lake ; a small quantity of iron has , r great 
influence on the shade. Weld contains more or less tannin, 
which gives very dark blue or green compounds with iron 
salts, the production of a very small quantity of which suffices 
to convert the yellow shade into an ugly dirty colour. 

Chalk may be used instead of soda to precipitate the 
alumina ; in this case the precipitate will contain calcium 
sulphate in addition to the alumina compound of luteolin. 

Weld lake may be used in oil, or size, or as a water-colour. 

Gamboge Lake, Crude gamboge is the dried sap of certain 
East Indian trees ; it is used as an artists' colour, but only in 
water ; for use in oil it is converted into a lake. A particular 
treatment is necessary to obtain a handsome lake. Gamboge 
is. treated for several days with water and the soft mass 
ground on the slab or in a mill until it forms a uniform 
paste, which is mixed with water to a thick liquid, which is 
then put through a fine sieve. A hot solution of alum is then 
added in the proportion of three parts of alum to one part of 



350 MINERAL AND LAKE PIGMENTS. 

gamboge, the mixture is boiled in a wooden vessel and two 
parts of nitric acid added whilst well stirring ; finally a dilute 
solution of potash is added in small quantities until the liquid 
solidifies to a jelly, which is spread out in thin layers on filter 
cloth, well washed with water and dried at a gentle heat. 

Prepared Gamboge. Crude gamboge cannot be used in 
oil painting because it never gives a uniform shade, but when 
the colouring matter is freed from admixtures and ground 
with oil, it gives a deep durable colour of great beauty. 

The colouring constituent of gamboge is a resin which is 
fairly soluble in strong alcohol, whilst the impurities are 
insoluble. The pure colouring matter is obtained without 
great expense in the following manner : Coarsely pow T dered 
gamboge is placed in a large flask with strong alcohol ; the 
flask is w T ell closed, placed in a warm position and repeatedly 
shaken ; the deep yellow solution is carefully poured off from 
the sediment, water is then added, when the colouring matter 
separates in flocks. Only sufficient water should be added to 
precipitate all the colouring matter ; test portions of the liquid 
should be examined from time to time, the alcohol can then 
be recovered by distillation and used again. The colouring 
matter separated from the alcoholic solution forms a hard 
mass on drying ; it must be very finely powdered before it is 
ground in oil. 

There are many other yellow lakes which can be obtained 
by adding alum to an aqueous decoction of the colour-bearing 
material and neutralising the liquid. Such decoctions are 
obtained from fustic, quercitron (Quercus tinctoria), young 
fustic (Bhus cotinus), the root of barberry (Berberis vulgaris), 
annato (the fruit of Bixa orrellana), turmeric (the root of 
Curcuma longa\ etc. The yellow lakes obtained from these 
sources are seldom used in painting, the mineral pigments 
are preferred on account of their finer appearance. These 
colouring matters are largely used in dyeing to produce 



YELLOW LAKES. 351 

shades varying from yellow to brown, the majority are also 
used in calico printing. 

Fustic Lake. Of the colouring materials mentioned above 
fustic produces a lake which is so handsome that it deserves 
larger use in painting than it has yet found. " Old fustic " is 
chipped immediately before use and extracted with boiling 
w r ater ; whilst still hot the extract is mixed with a hot solution 
of alum ; on cooling a fine yellow precipitate is formed, which 
when dry is frequently sold under the name of Dutch pink. 
(Genuine Dutch pink is made from a decoction of yellow 
berries ; the majority of the pigments sold under this name 
are made from decoctions of mixtures of the different yellow 
dye wares.) 

To obtain the best lake from fustic the alum used must 
be completely free from iron, otherwise only a dirty green 
lake is produced. Since the least trace of iron damages the 
shade of the colour, fustic lake is more frequently made 
by means of lead oxide. To obtain the lead fustic lake the 
decoction is allowed to stand for several days, so that the 
inorin, which together with maclurin forms the colouring 
principle of fustic, may separate ; the liquid is filtered from 
the precipitate, and lead acetate solution added which has 
been boiled with litharge to saturation, so that the most basic 
acetate is produced. The lead fustic lake has a deep yellow 
colour ; it is well adapted for use in painting, but, like all 
lead pigments, is not specially permanent in air. By admix- 
tures of levigated chalk or starch paler shades of the dull 
yellow lake are obtained. 

Quercitron Lake. Lakes are seldom made from quercitron 
alone, although they possess a deep yellow colour ; they are 
more frequently employed in dyeing when they are produced 
directly upon the fibre. Quercitron lake can be made by 
treating the aqueous decoction of the ground bark with tin 
crystals and a little alum solution. With tin crystals alone a. 



352 MINEEAL AND LAKE PIGMENTS. 

darker yellow lake is obtained than when alum is used at 
the same time. Different shades may thus be produced. 

Extract of quercitron bark comes into commerce as a 
greyish yellow powder under the name of flavine, which has 
great colouring .power, and may be used with advantage in 
the place of aqueous extracts of the bark. 

Since the discovery of the aniline dyes the applications of 
all lakes have been largely extended, since the dyes enable 
brighter shades to be obtained without the many processes 
necessary when natural colouring matters are used, and do 
not require so much skill on the part of the workman. This 
applies not only to yellow colouring matters, but to the many 
other colouring matters formerly used by the dyer. The 
principal reasons which have so quickly brought the new 
dyes to the front lie in their greater beauty and in the 
diminished labour required in their use. 

Purree or Indian Yellow must not be confounded with the 
yellow cobalt pigment known also under the latter name. It 
is a compound of magnesia with an organic acid euxanthic 
acid. It is obtained from the urine of cows fed upon mango 
leaves. It comes into commerce in lumps weighing about 
50 to 60 grammes, which are dark brown on the exterior, but 
on fracture show a fine orange yellow colour. Indian yellow 
is little used, and is not likely to attain importance. Many 
cheaper colouring matters equal in shade are known. 

The Colouring Matter of Saffron, The dried stigma of the 
flowers of the saffron, a species of iris, has been long used 
for colouring foods. It contains a very handsome yellow 
colouring matter of a characteristic shade ; formerly this was 
largely used in silk-dyeing, and especially for dyeing glove 
leather. It is practically now no longer used ; considerably 
cheaper aniline dyes produce an equally good shade. On 
account of its high price (1 kilogramme ofi saffron contains 
about 60,000 stigmata) this material cannot be used for lakes, 



YELLOW LAKES. 353 

although they leave nothing to be desired in fineness of shade, 
but they have no great permanence. 

The Colouring Matter of Gardinia Grandiflora, The fruits 
of this plant, which are imported from Southern Asia, contain 
a beautiful dark yellow colouring matter, which, according to 
Rochleder, is identical with the colouring matter of saffron. It 
is readily soluble in water ; lakes of various shades can readily 
be obtained by the addition of salts to the aqueous solution. 
Alum gives a pure yellow, lead acetate a yellowish red, and 
stannous chloride a dark orange red lake. 



23 



CHAPTEK L. 

KED LAKES. 
COCHINEAL AND CAKMINE. 

THE purple of the ancients was obtained from the shellfish 
jpurpura ; it was so costly that it was regarded as an attribute 
of royalty. According to history the Tyrians discovered this 
colour ; at any rate they were able to use it for dyeing. In 
the writings of the ancients Tyrian purple garments were 
regarded as the greatest luxury. The art of obtaining a 
purple from this source was lost ; more recently it has been 
re-discovered, but the colour is found in no way to correspond 
with what we regard as a bright shade. 

When the Spaniards conquered Mexico they found an 
insect cultivated which produced a splendid red colouring 
matter. This insect, the cochineal, is parasitic upon certain 
cacti, especially Cactus coccinellifer and Cactus opuntia. The 
females only are used as colouring material ; the males are 
very small, and much fewer in number. The cultivation of 
the proper species of cactus and of the cochineal has spread 
to most tropical countries. The females, which are attached 
in enormous numbers, are brushed off and killed by heating 
upon hot plates. They then appear as grains of the size of 
millet seeds, with a wrinkled surface covered with a silvery 
grey dust. This variety of cochineal is the best. Black 
cochineal also comes into commerce ; it has a brownish black 
colour. It is produced by killing the insects in boiling water, 



BED LAKES. 355 

by which the grey dust which covers the living insect is re- 
moved. Cochineal produces when ground an ugly reddish 
brown powder. 

The animal nature of cochineal cannot be recognised by 
the naked eye, it is seen on examination under the micro- 
scope ; this has given opportunity for the most incredible 
adulterations of the costly substance. Cases have been 
known in which a paste chiefly composed of flour has been 
pressed into the form of cochineal, the grains coloured by 
some cheap red colouring matter, powdered with the dust 
from boxes in which cochineal has been packed, and placed 
upon the market as cochineal. The expert would not be 
deceived by this gross fraud, but another method of adultera- 
tion is far more difficult to recognise ; genuine cochineal is 
brought into commerce from which the greater part of the 
colouring matter has been extracted, and which has been 
again powdered with the grey cochineal dust. Such extracted 
cochineal does not produce the same brownish red powder as 
the fresh substance. 

In the cochineal insect there is a very large quantity of 
colouring matter : it may reach 50 per cent, of the weight 
of the dry insect. The colouring matter, which is known as 
carmine (the same name is also applied to its lake), is soluble 
in water with a fine red colour. When cochineal is ex- 
tracted with water the operation must be often repeated, and 
each time fresh quantities of colouring matter are dissolved. 
The cochineal may be exhausted by repeated boiling with 
water, but a large quantity of liquid containing little colouring 
matter is then produced. In order to obtain a strong solu- 
tion of the colouring matter the cochineal must be powdered ; 
this is difficult on account of the softness of the material, 
which does not give a fine powder, but a pasty mass. The 
object is best attained by grinding the cochineal through 
a mill similar to a coffee mill, but since the grooves of the 



356 MINEEAL AND LAKE PIGMENTS. 

steel cone, which effects the crushing, may be easily clogged, 
the mill must be arranged so that the cone can be taken out 
and cleaned. 

The colouring principle of cochineal is an acid, carminic 
acid, which was obtained by its discoverer, Warren de la Hue, 
in the following manner : 1 part of powdered cochineal is 
boiled with 40 parts of water for 20 minutes ; after standing, 
the liquid is poured off from the sediment, and mixed with a 
solution of 6 parts of lead acetate, acidified by 1 part of acetic 
acid. The precipitate, which consists of impure lead carmin- 
ate, is filtered from the colourless liquid and carefully washed ; 
whilst still wet it is suspended in water, through which 
sulphuretted hydrogen is passed ; the lead carminate is decom- 
posed into lead sulphide and carminic acid, which dissolves in 
the water. The colouring matter is not yet quite pure ; the 
treatment with lead acetate and sulphuretted hydrogen must 
be repeated, the solution then obtained is evaporated at a 
low temperature, the residue dissolved in boiling alcohol, 
phosphoric acid added to decompose traces of lead carminate 
still present, then ether, finally the clear liquid is separated 
from the precipitate and evaporated. By this process carminic 
acid is obtained in a state of complete purity ; unfortunately 
the process is too complicated for practical application. Pure 
carminic acid is a purple red mass, which transmits red light 
at the edges and forms a pure scarlet red powder when 
ground. The aqueous solution gives, with alum and am- 
monia, a characteristic precipitate, the colour of which is the 
purest carmine red ; lead, zinc and copper salts produce 
purple red precipitates. The composition of carminic acid is 
expressed by the formula C 17 H 1S O 10 . 



CHAPTEK LI. 

THE MANUFACTURE OF CARMINE. 

THE colouring matter of cochineal is used in the form of a 
lake under the name of carmine, the finest and most expensive 
colour used in painting. However simple the preparation of 
carmine may appear, it is not easy to obtain a product of 
great beauty ; certain conditions, still unexplained, play an 
important part in the process. Until not long ago the manu- 
facture of fine carmine was regarded as a secret ; this it is no 
longer, and with proper care any one may produce an entirely 
satisfactory product. Many recipes have been given, some 
of the best will be mentioned. 

Whatever method is used to obtain carmine certain pre- 
cautions must be taken, without which it is impossible to 
obtain a bright colour. Alkalis and alkaline earths in very 
small quantity affect the shade of carmine, so that spring 
water should never be used in its preparation. Pure rain 
water, or, still better, distilled water, should be employed. 

The decoction of cochineal is difficult to filter. Paper 
cannot be used, because, the pores are so rapidly stopped up 
that new filters would be continually required. Fine silk 
is the most serviceable ; it should not be washed with soap 
the alkalinity of the small quantity of soap the fabric would 
retain would affect the colour. The decoction is made in a 
well-tinned copper pan, all other vessels should be of glass or 
porcelain, which are most easily cleaned, and great cleanli- 
ness is the prime essential to the success of the process. 



358 MINEKAL AND LAKE PIGMENTS. 

The greatest care is required to prevent the contact of the 
liquid with iron during the whole process, the smallest trace 
of this metal would result in a discoloured product. 

The essentials of the manufacture of carmine on the large 
scale are that the colouring matter is dissolved in water, and 
precipitated by the addition of an aluminium salt, generally 
alum, absolutely free from iron. The more slowly the 
carmine separates the finer is its colour. It is generally 
observed that the last portions to precipitate show the 
brightest shade. This is because the foreign substances 
occurring with the carminic acid in the decoction are thrown 
down with the first portions of the precipitate. In the pro- 
cess of Frau Cenette, famous for the beauty of the product, 
a solution is made from which the whole of the carmine is 
separated in about three weeks. During this long time the 
majority of the substances which have been dissolved in the 
water together with the carminic acid are decomposed ; the 
liquid acquires an unpleasant smell, and is covered with mould. 
In the author's opinion so good a product is obtained by this 
process because the greater part of the impurities is decom- 
posed : the nearer the product approaches the pure compound 
of carminic acid and alumina, the purer and brighter will be 
the shade of the carmine. 

It is known that light has considerable influence on the 
beauty of carmine. During dull winter days it is quite 
impossible to produce so fine a product as in summer. 
Instead of alum a tin solution may be used to precipitate 
the colouring matter ; the shade of the product is different 
to that of alumina carmine. 

Cenette's Method. 1 kilogramme of finely powdered cochi- 
neal is boiled with 75 litres of water for two hours ; 90 grammes 
of saltpetre are added, the liquid boiled for three minutes, 
then 120 grammes of salt of sorrel (acid potassium oxalate) 
are added and the liquid again boiled for ten minutes. The 



THE MANUFACTURE OF CABMINE. 359 

liquid is then completely clarified by standing, drawn off 
from the residue by a syphon, and brought into shallow glass 
dishes which are placed, protected from dust, in a bright light 
in a uniformly warm place. During several weeks the 
carmine separates, the last portions being always brighter 
than the first. The addition of potassium oxalate has the 
object of assisting the separation of .the carmine, for acid 
salts separate the carmine from solutions ; the saltpetre may 
reasonably be omitted. 

The majority of the recipes for carmine, which are fre- 
quently sold at a high price, differ but little from the above ; 
acid potassium tartrate is used instead of the oxalate, but the 
latter is to be preferred because of the slight solubility of the 
tartrate. It is important not to use too strong decoctions of 
cochineal, and to add only small quantities of alum. The 
clear liquid is placed in shallow glass dishes. After a few 
days the nature of the deposit should be examined : if a 
considerable quantity of a red precipitate has formed, the 
liquid is poured off into other dishes, in which carmine again 
separates during the following days, and usually of a brighter 
colour than the first. For example, 125 grammes of cochi- 
neal are boiled with 5 litres of water during fifteen minutes, 
30 grammes of very finely powdered alum are added to the 
boiling liquid, which is again boiled for a .few minutes, 
allowed to clarify and cool. The greater part of the carmine 
is then obtained in a few hours, but the liquid still separates 
carmine after several days. 

According to another formula 500 grammes of cochineal are 
boiled with 30 litres of water, 60 grammes of cream of tartar 
are added, then 30 grammes of alum, boiling is continued for 
several minutes, and the liquid then allowed to cool. Carmine 
may be obtained in a very short time by means of tin solution. 
The process is similar to that just given. The liquid which 
would be set aside for the spontaneous deposition of the 



360 MINERAL AND LAKE PIGMENTS. 

carmine is, however, returned to the pan, and a solution of 
pure stannous chloride added in drops, so long as the solution 
is still clearly red. The carmine separates at the bottom of 
the pan ; the liquid drawn off, even when it appears almost 
colourless, produces a further small quantity of carmine in a 
few days. 

According to J. J. Hess, a brighter carmine is obtained 
when the fat of the cochineal is previously extracted by ether 
or benzene. 

Proposals have often been made to deepen the colour of 
prepared carmine ; this must be done with the greatest care, 
for it is very easy to produce a less handsome instead of a 
finer pigment. The carmine is moistened with distilled water 
containing about 5 per cent, of ammonia solution. 

Carmine readily dissolves in ammonia. This property may 
be applied to test its purity, pure carmine should dissolve 
without residue in 5 to 6 times its quantity of ammonia. 
Any considerable residue denotes an intentional addition of 
some adulterant. Starch, vermilion, and cheaper lakes are 
used for this purpose. The red solution obtained by treating 
carmine with the above quantity of ammonia may be used as 
a red ink. This solution can also be used to purify carmine ; 
when it is allowed to stand in an uncorked bottle for some 
time the ammonia escapes, and the greater part of the carmine 
is deposited as a very fine powder. 

Carmine solution is made by dissolving carmine in the 
necessary quantity of ammonia, adding glycerine equal in 
quantity to the ammonia, driving off the latter by heat 
and diluting the liquid. This solution is well adapted for 
colouring confectionery, but cannot be employed in paint- 
ing and writing, since the glycerine would prevent it from 
drying. 

Carmine is extensively used in painting, and for many 
purposes cannot be replaced by another pigment, e.g., the 



THE MANUFACTURE OF CAEMINE. 361 

cosmetic known as vegetable rouge can be made from no 
other pigment. Carmine may be used in all methods of 
colouring ; it is quite harmless for confectioners' purposes. 

Munich, Vienna, Paris, or Florentine Lake. This fine 
deep red lake differs from carmine in containing a much 
larger quantity of alumina, thus possessing the character of 
an ordinary lake. Occasionally carmine lake is intentionally 
mixed with light white substances, such as magnesia, to 
obtain paler shades or cheaper pigments. 

Whilst only the finest varieties of cochineal can be used 
to make fine carmine, the cheaper sorts can be used for 
Florentine lake. The cochineal used for carmine is boiled 
but a short time with water, so that the residue contains con- 
siderable quantities of colouring matter, which may amount 
to half of that originally present. The same materials are 
used as in the preparation of carmine, and in this case, too, 
stress should be laid upon their purity. The weight of the 
alum is usually 10 to 15 times that of the cochineal ; a little 
stannous chloride and cream of tartar are also added to 
brighten the shade. All these materials are boiled with /the 
cochineal, soda is added to the clear solution until effervescence 
no longer occurs, the separated lake is then washed. The 
lake may be made with magnesia instead of alumina, it is 
added in the form of magnesium sulphate ; the more magnesia 
is used the paler is the lake. The proportions in which the 
materials are employed vary with each manufacturer. The 
following quantities have always given the author a favourable 
result : Cochineal 10 parts, alum 150 parts, water 250 parts ; 
or cochineal 10 parts, magnesium sulphate 5 parts, alum 0'5 
part. 

In making Florentine lake a fairly deep colour should 
be produced. This may be mixed without difficulty in the 
wet or the dry state with a white pigment, thus producing any 
desired shade, even to the most delicate rose red. 



362 MINEKAL AND LAKE PIGMENTS. 

Ammonia-Cochineal, This preparation, which is chiefly 
used by dyers, is obtained by treating cochineal in a well- 
closed flask with strong ammonia, which dissolves the colour- 
ing matter. After about a month alum equal to about 3 per 
cent, of the amount of cochineal is added without separating 
the undissolved residue from the solution, and the whole is 
evaporated at a gentle heat in a tinned pan until it becomes- 
a stiff paste on cooling. The mass solidifies more readily if a 
small quantity of starch paste is added. " Cochineal paste " 
is cochineal which has been treated in this manner ; it is 
generally brought into the market in the form of cakes or 
small slabs. 

In addition to the true cochineal other species of the same 
insect contain a red colouring matter and have a restricted 
use in dyeing. The most important of these is the Polish 
cochineal (Coccus polonicus), which lives on the roots of the 
scleranthus. It has not been proved that these insects con- 
tain the same colouring matter as true cochineal, but it is 
certain that the colouring matters of Polish and also of 
Bussian cochineal (Coccus euvce ursi) are far inferior in beauty 
to that of true cochineal. 



CHAPTEK LII. 

THE COLOURING MATTER OF LAC. 

FROM the punctures of the insect Coccus lacca on certain East 
Indian trees, especially those belonging to the genus Ficus, 
flow at the same time resin and colouring matter in such 
quantities that the insects are frequently enclosed and large 
red masses are formed on the trees. The insects live in some 
measure in these masses, the females lay their eggs in the 
spaces and the larvae are said to feed on the red sap con- 
tained in the mass. When the larvae have left the incrusta- 
tion, generally in November, it is broken off from the 
branches and collected. It is now known as stick-lac ; it 
consists of large masses of resin of a fine deep red colour. 
In the interior the cells of the insects may still be perceived. 
On chewing, it becomes soft and colours the saliva a deep 
violet. When stick-lac is boiled in water a portion of the 
colouring matter dissolves. Good stick-lac contains 10 per 
cent, of colouring matter and 80 per cent of resin. 

Seed-lac has the same origin as stick-lac, but frequently 
the greater part of the colouring matter has been extracted ; it 
rarely contains more than 2'5 per cent., and is consequently 
of little value for colouring purposes. 

Lac Dye. The colouring matter of stick-lac is generally 
separated from the resin in India and comes into the market 
under the name .of lac dye. It is made by stirring coarsely 
powdered stick-lac in large vessels for several hours with 
warm water. Almost the whole of the colouring matter 



364 MINERAL AND LAKE PIGMENTS. 

dissolves, whilst the resin remains as a ruby red mass which 
is melted and brought' into the market under the name of 
shellac. The solution of the colouring matter is evaporated 
in shallow vessels in the sun or boiled down in pans ; the 
residue is made into cakes. At present soda solution is used 
instead of pure water to extract the colouring matter ; a larger 
yield is obtained. Lac dye usually contains 45 to 50 per cent, 
of colouring matter, 25 per cent, of resin, and, in addition, 
earthy substances which are to be regarded as intentional 
admixtures. 

In the preparation of lac dye by Stephen's method 
coarsely powdered stick-lac is boiled with soda, and the 
remainder of the colouring matter contained in the resin 
extracted by repeated boiling with water. All the extracts 
are then united and precipitated by alum. The colouring 
matter separates as a lake which still contains a large 
quantity of resin. 

According to the patented process of Henley, the colour- 
ing matter of seed-lac is extracted by hydraulic presses. The 
lac is filled into press bags which are placed between iron 
boxes heated by steam. On applying pressure the melted 
resin, shellac, comes through the bags, whilst the colouring 
matter remains behind. If this method were employed on 
the large scale, presses similar to those used in stearin candle 
works would be suitable. 

When lac dye is treated with hydrochloric acid the colour- 
ing matter dissolves ; the solution can be used to dye wool, 
which it colours a beautiful red. The colouring matter is 
very similar in appearance to that of cochineal, but has the 
appearance of much greater fastness. It appears also to be 
similar in composition to carminic acid, but little is known 
as to its chemical constitution. 

When lac dye is to be used for painting it must be freed 
from resin ; this is accomplished by treating the finely 



THE COLOUEING MATTEE OF LAC. 365 

powdered mass for a long time with boiling alcohol and 
separating the solution of resin from the undissolved colour- 
ing matter, which is then dried. In this way a lake is pro- 
duced known commercially as Vienna red and little inferior 
to carmine lake. Unfortunately the price of lac dye, 
although it is imported in large quantities from India to 
England, is so high that it can only be used for artists'" 
colours. 



CHAPTEE LIIL 

SAFFLOWER OR CARTHAMINE RED. 

THE safflower or bastard saffron (Carthamus tinctoria) grows 
wild in Southern Europe, and is also cultivated. It contains 
two colouring matters, yellow and red. The former is not 
used in dyeing, but is employed for colouring liqueurs, since 
it is innocuous. The red colouring matter is used for colour- 
ing artificial flowers and for fine cosmetics ; formerly it 
was employed in dyeing, but is now rarely used for this 
purpose, since carthamine red has little permanence and 
can be replaced by cheaper dyes. 

The yellow colouring matter, which is not used, is 
generally removed by treating the corolla of the safflower, 
which contains the colouring matter, with water. After this 
treatment it is known as washed safflower. In the dried 
corolla of the flower up to 36 per cent, of the yellow 
colouring matter is found, whilst only 0'4 to O6 per cent, 
of the red is contained. 

It appears that the colouring matter of the samow r er has 
been used from the most ancient times. The Chinese em- 
ploy it to obtain a handsome cosmetic, the Tyrians are said 
to have used it in dyeing. In Europe it was not cultivated 
before the seventeenth century, when it came into use for 
dyeing. 

In order to obtain pure carthamine red and this is the 
form in which it is generally used safflower is treated for 



SAFFLOWER OR CAKTHAMINE BED. 367 

a long time with water containing a little acetic acid until 
a yellow solution no longer results. The residue is then 
treated with soda solution for several hours, when the colour- 
ing matter dissolves ; the solution is filtered and neutralised 
with acetic acid. Cotton is then introduced. Carthamine 
red is a colouring matter which at once dyes animal and 
vegetable fibres when these are brought in contact with its 
solution. Thus the whole of the colouring matter is pre- 
cipitated on the cotton, which is coloured a deep red. After 
twenty-four hours the cotton is taken out, washed, and 
treated with soda solution, in which the colouring matter 
dissolves. When this solution is carefully neutralised with 
citric acid the colouring matter separates in the form of fine 
flocks. These are collected, dissolved in strong alcohol, the 
solution evaporated to a small volume, the colouring matter 
again precipitated by the addition of a large quantity of water, 
and washed with pure water until the wash waters begin 
to be coloured red. The pure carthamine thus obtained is 
spread upon small cups, in which it dries to a beautiful red 
mass which in somewhat thicker layers has a fine green 
lustre. It is brought into commerce as "cup red" or "plate 
red". 

The small quantity of colouring matter contained in the 
safflower, as well as the complicated process for obtaining 
it, make it evident that this colouring matter will be among 
the most expensive found in commerce. The high price is, 
however, neutralised by its great colouring power, and by 
the fact that nothing can in all cases replace it for colouring 
the best artificial flowers. 

Safflower Carmine. This substance, used by dyers, is a 
solution of carthamine red in soda solution. It is only 
necessary to add an acid after once introducing the fabric 
in order to fix the colouring matter at once upon the fibre. 

The shades produced by carthamine red are distinguished 



368 MINEBAL AND LAKE PIGMENTS. 

by a delicacy of tint not produced by other dyes ; all shades 
between the deepest red and the palest rose red may be 
obtained from it. Unfortunately it is very susceptible to 
the action of alkalis ; one careless washing of an article dyed 
with carthamine is sufficient to remove the greater part of 
the colour. 

On account of its cost and little durability carthamine 
red is not used in painting. It is principally employed in 
the artificial flower industry for colouring flowers. Pure 
carthamine or a mixture with finely powdered steatite is 
applied by rubbing. 

Alkanet. The roots of alkanet (Alkanna tinctoria) con- 
tain in the bark a fine red colouring matter, from which also 
a violet lake can be obtained, though this is rarely made. 
The plant is largely grown in Southern Europe and also 
around Vienna. The colouring matter is obtained pure by 
macerating the root for a long time with water and then 
treating with strong alcohol, which dissolves the colouring 
matter, together with a large quantity of resin. The alcohol 
is distilled off, the residue extracted with ether, and the 
ethereal extract treated with a large quantity of water, 
which extracts the colouring matter. This is left in the 
pure state, when the solution is slowly evaporated. 

According to Carnelutti and R. Nasini alkannin is obtained 
by extracting the root of Anclmsa tinctoria with petroleum 
ether, evaporating, treating the residue with weak caustic 
potash, filtering, and shaking the filtrate several times with 
ether. The solvent is removed, the colouring matter precipi- 
tated by carbonic acid, dried over sulphuric acid, and dis- 
solved in ether. The filtered solution is then allowed to 
evaporate. Alkannin is then obtained as a dark brownish 
red mass, which is easily powdered. It has a metallic lustre, 
and readily dissolves in alcohol, glacial acetic acid and 
chloroform. 



SAFFLOWEE OE CAETHAMINE EED. 369 

When alum is added to the aqueous solution of the colour- 
ing matter a beautiful violet precipitate separates, which when 
dry is very suitable for artistic purposes. Alkanet is little 
used. It is completely replaced by much cheaper colouring 
matters. 



CHAPTEK LIV. 

MADDER AND ITS COLOURING MATTERS. 

THE root of the madder (Bubia tinctoria) contains a red 
colouring matter which is distinguished over all other colours 
of vegetable origin by great fastness, on account of which it 
occupies a most important position in dyeing and colour- 
making. The use of madder in dyeing has been much re- 
stricted by the discovery of artificial methods of making 
alizarine, the most important colouring matter of madder. 
Thus the following account of madder and its products is 
chiefly of historical interest only. 

Madder is cultivated in many countries. The roots, which 
vary from the thickness of a quill to that of a finger, are 
cleansed from adherent earth, carefully sorted according to 
size, and dried. In commerce a distinction is made between 
stripped and unstripped madder. The cortex of the root con- 
tains but little colouring matter and is generally removed by 
mill stones moving at some distance apart. The stripped 
roots are finely ground, and are then known commercially as 
madder. Merchants distinguish a large number of varieties, 
which are designated according to the place of production. 
In addition to ground madder there are a number of madder 
preparations which contain the colouring matter in concen- 
trated form, and are consequently used in smaller quantity. 
The most important of these preparations are garancin, 
garanceux, and the madder extracts. 

" Kefined madder " is made from the natural substance 



MADDER AND ITS COLOURING MATTERS. 371 

by subjecting it to a process of fermentation, by which the 
substances accompanying the colouring matters are largely 
decomposed, and the product becomes relatively richer in 
colouring matter. 

" Flowers of madder," which have double the colouring 
power of madder, are made by treatment with very dilute 
sulphuric acid. Madder is stirred with five or six times its 
quantity of water to which about 1 per cent, of sulphuric 
acid has been added. After leaving the mixture in a fairly 
warm place for five to six days alcoholic fermentation takes 
place, and many substances which would be injurious in dyeing 
are decomposed. The residue becomes in consequence richer 
in colouring matter. When the fermentation is finished 
the residue is filtered off, subjected to powerful hydraulic 
pressure, and the mass broken up and very thoroughly 
dried at a temperature of 60 to 70 C. If it is not thoroughly 
dried it soon becomes mouldy. This method of treating 
madder, due to Julians, is simple, and has the advantage that 
from the alcoholic liquid spirit may be obtained. In some 
respects this process is more rational than the one immedi- 
ately following : certain substances which are destroyed in 
the manufacture of garancin are made useful, and also the 
process requires little outlay. 

Garancin is obtained from madder by treatment with sul- 
phuric acid, which decomposes the glucoside, and also sets 
free the colouring matter united to lime and magnesia, which 
would otherwise be lost. In addition, the warm acid radically 
attacks the nitrogenous compounds, the greater portion of 
which is destroyed. Thus the residue contains relatively 
more colouring matter, and the removal of substances which 
accompany the colouring matter considerably facilitates the 
dyeing process. 

Although the preparation of garancin was recommended 
in 1828, it was much later before the prejudice which pre- 



372 MINERAL AND LAKE PIGMENTS. 

vented its general employment was completely overcome, 
and garancin recognised as a very valuable material for the 
dyer. The simplest process for preparing garancin is as 
follows : Madder is several times washed with water, then 
pressed, and the residue mixed with dilute sulphuric acid in 
a lead-lined vessel. To 100 parts of madder 50 parts of sul- 
phuric acid and 50 parts of water are taken, the mixture is 
heated by steam to 100 C., and maintained at this temperature 
for half an hour. By the action of sulphuric acid of this 
strength considerable charring occurs, and in particular the 
cell walls of the madder are attacked, so that in the suc- 
ceeding process the colouring matter is readily dissolved ; 
in consequence o,f the charring, the garancin acquires a 
deep brown to black colour. When the action of the acid 
is finished, it is drawn off and the residue washed with water 
until it is free from acid, when it is dried. The process here 
described is the original method ; it has been much modified, 
hydrochloric acid, zinc chloride, alkalis, soap solution, etc., 
being used in the manufacture of garancin. The colour maker 
will rarely require to make garancin himself, so that it is 
sufficient for our purpose to indicate what is understood by 
garancin, and in what manner it is made. 

Garanceux. Madder which has been once used in dyeing 
still retains some quantity of colouring matter ; this is utilised 
by treating it by the same process which is used to obtain 
garancin from fresh madder. The treatment with sulphuric 
acid almost entirely destroys the cellular structure, so that 
the residual colouring matter is made accessible to solvents, 
and consequently the garanceux may be again used for 
dyeing. 

Madder Extract The fixation of the colouring matter of 
madder upon the fibre is attended w r ith many difficulties ; 
attempts were made to present it in the purest form possible, 
or in such a condition that the operation of dyeing w T as 



MADDEE AND ITS COLOURING MATTERS. 373 

simplified. The commercial madder extracts consist either 
of liquid extracts of madder, or of the more or less pure 
colouring matter itself. Among the numerous solid or liquid 
madder extracts, without doubt the most important is the 
crude alizarin (the principal colouring matter of madder), 
made by the process discovered almost simultaneously by 
Rochleder and Pernod. This is produced by extracting either 
madder or garancin with hot water containing sulphuric 
acid ; about 5 grammes of acid are mixed with 1 kilogramme of 
water and the madder boiled with this dilute acid in a lead- 
lined vessel. The liquid separated from the solid residue 
becomes turbid on cooling, and yellow flocks separate which 
consist of impure alizarin ; no further purification is required 
for practical purposes. Crude alizarin is also obtained by 
treating madder with superheated steam. These pre 
parations are important, not only to the dyer and calico 
printer, but also to the colour maker ; from them the madder 
pigments can be made in a very simple manner. 

The Constituents of Madder, Madder has been most 
thoroughly investigated ; its colouring matter, alizarin, is now 
made artificially. In addition to woody fibre, madder contains 
sugar, mucilage, resin, a glucoside (i.e., a substance which can 
be decomposed into a sugar and another substance), and, in 
particular, two colouring matters known as alizarin and 
purpurin. 

Alizarin occurs ready formed in madder, and is also 
produced by the decomposition of the madder glucoside into 
alizarin and sugar. To obtain pure alizarin finely ground 
madder is extracted with boiling water ; on the addition of 
sulphuric acid to the decoction alizarin separates together 
with other substances. The moist precipitate is boiled with 
a solution of aluminium chloride, and, after filtering, hydro- 
chloric acid is added to the filtrate, when deep red flocks 
separate, consisting of a mixture of alizarin and purpurin. 



374 MINEEAL AND LAKE PIGMENTS. 

The colouring matters are further purified by dissolving in 
alcohol and adding freshly precipitated alumina, which unites 
with them ; the alumina compounds are then boiled with 
strong soda solution, in which the purpurin dissolves. The 
residue consists of aluminium alizarinate mixed with resin, 
the latter is extracted with ether or benzene, and the residue 
then decomposed with hydrochloric acid, when alizarin is set 
free and is purified by recrystallisation. 

Alizarin has the formula C ]4 H,.O 2 (C)H).>. It forms fine 
red crystals which are very little soluble in cold but more 
easily in hot water ; they dissolve readily in strong alcohol to 
a yellow solution. Alizarin dissolves in alkaline liquids, the 
solution is dichroic ; it appears dark purple by transmitted 
light and pure blue by reflected light. When the alkaline 
solution of alizarin is brought in contact with freshly pre- 
cipitated alumina the colouring matter is completely thrown 
dow T n and a beautiful red lake is formed. 

The second madder colouring matter is purpurin. This 
is more soluble in water than alizarin, it also dissolves in 
alum solutions ; its alkaline solutions are not dichroic. The 
formula of purpurin is C 14 H,-0 2 (OH) a . In addition to these 
two colouring matters, madder contains a third, known as 
rubiacin, which also forms red compounds. It appears to be 
fixed upon fabrics dyed with madder in company with the 
other colouring matters. 

Dyeing and printing with the colouring matters of madder 
are among the most difficult processes in dyeing ; a long 
series of operations is required to fix the colour permanently 
on the fabric. The beautiful deep red colour known as 
Turkey red is produced by madder ; it is distinguished from 
other vegetable colours by its great fastness. By means of 
madder various other shades besides pure red may be pro- 
duced. This very valuable substance is largely used in dyeing 
and calico printing to produce permanent colours. 



CHAPTEK LV. 

MADDER LAKES. 

THE madder lakes are equally as permanent as the madder 
colouring matters dyed upon fabrics. On this account and 
because of their handsome colour they are highly prized by 
painters. The best qualities of madder are necessary to 
produce fine lakes ; it is more convenient to use garancin or 
madder extract. Only when the colouring matter' is tolerably 
free from the foreign substances which accompany it can a 
pure red madder lake be produced ; the poorer qualities of 
madder produce a lake which is not a pure red and is never 
bright. The good repute of the madder lakes of certain makers 
is in great part due to careful choice of the raw materials. 
This choice and care in the process are the secrets of the 
manufacturers who produce good madder lakes. 

The author has found that the finest lakes can be ob- 
tained without difficulty from the alizarin, separated in the 
before-mentioned manner from madder by means of sulphuric 
acid. The small cost of obtaining crude alizarin by this 
process enables it to be used on a commercial scale. The 
labour involved in the process is far outweighed by the 
excellent qualities of the lakes produced. / 

A very bright lake is obtained directly from madder by 
treating with sulphuric acid (thus garancin is really used), 
digesting the mass for several hours with a solution of alum 
absolutely free from iron, filtering and adding at first a small 
quantity of soda solution ; a little madder lake is precipitated 



376 MINEKAL AND LAKE PIGMENTS. 

which is of the finest shade. When this lake has settled 
the liquid is poured off and again mixed with a little soda 
solution. The lake now precipitated is inferior to the first, 
if the madder is not of specially fine quality. By this 
fractional precipitation of the colouring matter, madder lakes 
varying in beauty are produced ; the separate fractions often 
differ considerably in shade, this is because the later portions 
are more and more contaminated by the foreign substances 
contained in the madder. 

From inferior madder lake, which could only be sold at a 
low price, a lake of the best quality can be produced in the 
following manner : The lake is finely powdered, mixed with 
acetic acid and left to stand twenty-four hours ; the madder 
lake dissolves in the acid to a fine red solution, whilst the 
impurities remain on the filter when the solution is filtered. 
The clear solution is mixed with a large volume of water 
free from lime, and the acetic acid neutralised by soda, when 
pure madder lake separates. The addition of soda is not 
continued until all the lake is precipitated, but only until the 
liquid is still slightly red. Hydrochloric acid may be used 
instead of acetic ; it is much cheaper, but must be absolutely 
free from iron. 

It has already been stated that the crude alizarin prepared 
as described above is well suited as a source of madder lake, 
since it is free from the majority of the substances which 
injure the purity of the shade. It is treated with a boiling 
solution of alum and filtered whilst hot: madder lake is then 
precipitated by cautious additions of soda, the first portions 
precipitated being the best. 

Madder Carmine is not often found in commerce ; it con- 
sists of the almost pure lakes of alizarin and purpurin. The 
process by which this valuable pigment is produced is based 
upon the great stability of the madder colouring matters. 
The following is the process on the large scale : Good madder, 



MADDER LAKES. 377 

very finely ground, is spread out in small heaps in a room, 
the temperature of which is about 16 to 18 C. ; the heaps are 
moistened with water and left for several days. The mass 
ferments and gives off a peculiar smell ; in the fermentation 
not only is the glucoside decomposed, but also many of the 
other compounds contained in the madder, consequently the 
mass becomes dark and considerably diminishes in weight. 
The end of the fermentation process may be recognised by a 
little practice with tolerabte certainty by the disappearance 
of the peculiar smell ; the mass is broken up and transferred 
to a lead-lined vessel, in which it is mixed with three or four 
times its weight of ordinary sulphuric acid. The acid is 
allowed to act for several hours, the plant fibres are almost 
completely charred, and the mixture becomes black. When 
the charred residue has settled, the liquid is filtered through 
pure quartz sand or powdered glass, and mixed with a large 
quantity of water. The carmine, which is insoluble in water, 
separates as a red powder ; it is then washed and dried. 

The colour of madder carmine is of such beauty that it 
can only be compared with that of good cochineal carmine, 
but it is incomparably more permanent than the latter and 
may be uked in all varieties of painting. 



CHAPTER LVI. 

MANJIT (INDIAN MADDER). 

THE roots of the East Indian Bubia mungista contain purpurin 
and another characteristic colouring matter, manjistin. This 
material is a rarity in German commerce, but is imported 
into England from India in tolerable amount and used in 
dyeing. Manjistin may be extracted by boiling man jit re- 
peatedly with a solution of aluminium sulphate, uniting the 
liquids and acidifying strongly with hydrochloric acid. In 
twenty-four hours a red precipitate forms in the liquid ; it 
is dried and treated with boiling carbon disulphide, which 
extracts purpurin and manjistin and leaves undissolved a dark 
resin. The residue, after distilling off the carbon bisulphide, 
gives up purpurin to dilute acetic acid, manjistin remain- 
ing undissolved. When the purpurin has been completely 
extracted, the residue is treated with a little boiling alcohol and 
repeatedly recrystallised ; manjistin is finally obtained in golden 
yellow crystals of the composition C 14 H 15 0.,(OH) 2 . CO.,H. 

When a solution of manjistin in a little alcohol is mixed 
with water no precipitate occurs ; on then adding aluminium 
hydrate and long boiling an orange-red lake is produced. 
Lead acetate produces a deep orange-red lake when used in 
just sufficient quantity ; with excess a fine scarlet lake is ob- 
tained. 

If manjit were a common article in the German market, 
it would be used in the preparation of fine pigments with as 



MAN JIT (INDIAN MADDER). 379 

much advantage as madder and the various madder prepara- 
tions. 

Chica Red, Curucuru, Carajuru, The colouring matter 
known under these names rarely comes into commerce ; it 
forms brownish red masses which acquire a peculiar golden 
glitter on rubbing. It is obtained from the leaves of Bignonia 
chica, a native of tropical America. The leaves are super- 
ficially dried and then covered with water ; in the warm 
climate the macerated leaves soon ferment, and a deep red 
powder separates at the bottom of the vessel : this is the 
colouring matter ; it is dried, made into small cakes and 
brought into the market. 

Chica red is only partially soluble in strong alcohol, the 
residue chiefly consists of plant cells. Alum and stannous 
chloride precipitate beautiful red lakes from the solution, which 
are distinguished by great permanence when exposed to light. 
On account of the present high price, this colouring matter 
has not yet obtained admission into the colour industries, 
though it would be a valuable recruit. 

Chica red has been little studied ; an investigation would 
certainly considerably increase our knowledge of the organic 
colouring matters. Towards reducing agents it behaves in 
a similar manner to indigo ; in contact w T ith caustic soda, 
grape sugar and water in closed flasks, a violet liquid is 
produced ; when exposed in an open vessel the violet at once 
changes to brown, but if the liquid is syphoned off into dilute 
hydrochloric acid a pure red precipitate separates, which is to 
be regarded as the pure colouring matter. 



CHAPTER LVII. 

LICHEN COLOURING MATTERS. 

MANY lichens contain compounds from which colouring 
matters can be obtained by appropriate treatment ; the 
colouring matters known as cudbear, archil and litmus were 
formerly much more extensively used in dyeing than at 
present : like so many natural colouring matters they have 
been replaced by coal-tar dyes. 

The lichens which produce colouring matters live upon 
trees and rocks, chiefly on the sea-shore ; the colouring 
matters are thus generally manufactured in places near the 
coast. The species principally used for this purpose are 
Lecanom, Eocella and Usnea. 

The lichens contain colourless compounds, which by 
treatment with acid or alkalis produce orcinol. In contact 
with moist air and ammonia, orcinol is decomposed into 
water and orceiin, which is the colouring matter. The lichens 
are treated so as to produce the largest quantity of orcinol, 
which is then converted into orceiin. There is not space here 
to enumerate and describe the many compounds obtained 
from lichens, only the most important can be mentioned; 
these are erythric, lecanoric and usnic acids. Lecanoric acid 
produces, under certain conditions, orsellic acid, which is again 
changed to orcinol. 

Pure orcinol, C 7 H 8 9 . H 2 O, is a crystalline substance of 
.sweet taste ; it is soluble in ether, alcohol, water, etc. In 



LICHEN COLOUKING MATTERS. 381 

damp air containing ammonia, orcinol gradually acquires 
a fine red colour, being converted into orcei'n, C 7 H 7 N0 3 , 
according to the following reaction : 

C 7 H 8 2 + NH 3 + 30 - C 7 H 7 N0 3 + 2H 2 0. 

When pure, orcei'n is a brownish red amorphous powder, 
soluble in alcohol to a scarlet solution and in alkalis to 
a purple solution. On the addition of water to the am- 
moniacal solution or of salt to the alkaline solution, orcein is 
again separated ; this procedure may be used to completely 
purify the colouring matter. 

Archil is principally obtained from the lichens Lecanora 
and Eocella. These lichens have a wide geographical distribu- 
tion ; archil is made in Sweden, Italy and Spain. It is 
obtained in a simple manner : the powdered lichen is mixed 
with decomposing urine to a paste, which is allowed to lie in 
the air until the colour has changed through red to violet ; on 
the average fourteen days are required for the transformation. 
More recently the process has been made less disgusting by 
replacing urine by ammoniacal gas liquor. When the mixture 
is violet all through the process is finished ; the wet mass 
is packed into barrels, which are well closed to prevent it 
from drying. 

The presence of alkalis accelerates the formation of orcein ; 
in many districts it is customary to add a little lime to the 
mixture of lichen and urine ; alum is also added to retard 
putrefaction, which readily occurs in warm climates. 

Instead of using lichens to produce the colouring matter, 
they may be extracted by boiling water, the solution con- 
siderably evaporated and then exposed to the air with the 
addition of ammonia, until the violet colouration appears. 
Slight additions of sulphuric acid give a more purple tint ; 
soda produces a deeper violet. 

French Purple is a compound of the archil colouring 
matter with lime, i.e., a lime lake. This beautiful substance 



382 MINERAL AND LAKE PIGMENTS. 

is obtained by treating the lichen for some minutes with 
ammonia, pressing out the liquid and neutralising it exactly 
with hydrochloric acid. The precipitate obtained consists of 
the lichen acids ; it is again dissolved in ammonia and the 
solution exposed to the air until it has become cherry red. 
When this is the case the liquid is quickly boiled and placed 
in large shallow dishes, which are kept at a temperature of 
75 C. until the colour is purple- violet. Calcium chloride is 
then added ; a garnet red precipitate is produced, which, 
when washed and dried, constitutes French purple. 

Archil and French purple produce shades of great beauty 
and purity, but they have little durability and are quickly 
altered by exposure to light. They are, therefore, seldom 
used as pigments. The same is the case with cudbear and 
litmus. 

Cudbear is a dirty purple powder with an ammoniacal 
odour and salt taste. It is made on a large scale in Holland. 
It differs from archil only in containing the colouring matter 
in the solid form. 

Litmus is generally made from lichens of the genus Vario- 
laria, though varieties of Rocella and Lecanora are often used. 
It is not made in quite the same manner as archil and 
cudbear. Together with the ammonia, potash is added to 
the powdered lichen. The mixture is left until it has become 
violet, when urine, potash and lime are added at intervals to 
continue the decomposition until the colour is changed to 
blue and the whole mass has become a paste. Gypsum and 
lime are then added, the mixture formed into rectangular 
plates, and brought into the market, described by a number, 
according to the amount of gypsum added. 

Pure litmus should leave little solid residue when dis- 
solved in water, the solution should be a fine violet. On 
account of its want of stability this colouring matter is not 
used in dyeing. It can be used for colouring foods since it is 



LICHEN COLOURING MATTERS. 383 

innocuous ; but indigo carmine has much greater colouring 
power and at the same time gives fine shades. 

The only use of litmus at the present time is as an indi- 
cator to decide whether a solution is acid, neutral or alkaline. 
In a quite neutral solution litmus is violet. Acids turn the 
solution to red and alkalis to pure blue. In preparing litmus 
tincture regard must be given to the fact that the aqueous 
solution of the colouring matter is always alkaline ; acid 
must be cautiously added until the colour is violet, and the 
least addition of acid or alkali suffices to change it to red or 
blue. 



CHAPTEK LVIIL 

RED WOOD LAKES. 

EED wood is obtained from species of Ccesalpinia growing in 
South America. The following varieties are distinguished : 
Fernambuco or Brazil wood, Bahia wood, St. Martha wood, 
Lima wood and Sapan wood, all of which contain the same 
colouring matter, but in different quantities. Bed wood 
comes into commerce either as thick logs of a fine red 
colour or as finely rasped chips. Fernambuco and Sapan 
woods are accounted the best varieties. Although the 
colouring matters contained in these woods have been 
investigated for a long time, an accurate knowledge of them 
has only recently been obtained. The pure colouring matter 
of red wood, brasilin, forms orange-coloured needle-shaped 
crystals, soluble in water, alcohol and ether. On careful 
heating it partially sublimes undecomposed. Brasilin most 
probably results from the alteration of a substance contained 
in the inner less coloured portion of the wood ; this com- 
pound, brasilei'n, has been obtained in colourless needles ; its 
solution acquires a red colour on boiling, and produces the 
pure colouring matter. 

Brasilin is a weak acid of the composition C 16 H 14 5 . It 
forms handsomely coloured compounds with most metallic 
oxides, which are much used in dyeing. A decoction of red 
wood gives with alum and soda solution a handsome lake of 
a pure red colour. With a solution of stannic chloride it at 
once 'produces a red precipitate. When a solution of potas- 



RED WOOD LAKES. 385 

slum chromate is added to a decoction of redwood, a dark 
brown precipitate separates, which is a compound of chrom- 
ium oxide with the partially altered colouring matter. The 
decoction produces a brownish red compound with ferric 
oxide. 

The decoctions of red wood contain, in addition to the 
colouring matter, a number of other substances which would 
injure the shade of the lake obtained from the decoction. 
The greater portion of these substances may be separated by 
allowing the decoction to stand for several days, when a dirty 
reddish brown mud forms at the bottom of the vessel. This 
simple method of purification is attended with the dis- 
advantage that, especially in summer, the decoction rapidly 
becomes mouldy. This may be prevented by the addition of 
a little carbolic acid ; not more than O'Ol per cent, of the 
quantity of liquid is required. The foreign matters may also 
be separated by adding milk or a solution of glue to the decoc- 
tion, but the simpler process of allowing it to stand gives the 
best result. 

From the behaviour of brasilin towards the metallic oxides, 
as given above, it follows that compounds of different colours 
can be made from red wood. These are utilised in dyeing, but 
in colour works red wood is only used for the preparation of 
red lakes. The lakes are found in commerce under numerous 
names, the most common of which are Venetian lake, Vienna 
lake, Florentine lake, Berlin lake, etc. According as the 
colouring matter is 1 " combined with alumina alone or with 
stannic chloride in addition, pale or deep lakes are obtained. 
The different shades of red wood lakes found in commerce are 
obtained by additions of a white pigment. For this purpose 
levigated chalk, gypsum, lycopodium powder, and other white 
pigments light in weight are used. 

In preparing red wood lakes the decoction should always 
be purified by the process mentioned above ; there is then more 

25 



386 MINERAL AND LAKE PIGMENTS. 

certainty of obtaining a bright lake. It is advisable to 
dilute the decoction considerably with pure water before pre- 
cipitating the lake ; experience has shown that brighter colours 
are thus obtained. To 100 parts of wood 130 to 150 parts of 
alum are used. The latter is dissolved alone and mixed with 
the decoction, the lake is then precipitated by soda solution. 
As the lake separates the liquid becomes lighter ; portions 
should be taken out of the vessel in order to see when all the 
colouring matter is thrown down. If too much soda solution 
is added the lake takes a violet shade. 

A particularly fine lake is obtained from Fernambuco wood 
when the extract is acidified with a weak acid, such as acetic, 
and stannic chloride used together with alum in precipitating 
the lake. One hundred parts of rasped wood are boiled with 
300 parts of water to which acetic acid has been added ; the 
extract is then boiled with 130 parts of alum until the latter 
is dissolved. To the filtered liquid 20 to 25 parts of stannic 
chloride are added whilst stirring well, and the lake is at 
once precipitated by soda solution. 

If the lake from Fernambuco wood does not turn out 
well, a better product may be obtained by treating the lake 
with such a quantity of hydrochloric acid that a small portion 
remains undecomposed. This is filtered off, and the colour- 
ing matter reprecipitated in combination with alumina by 
neutralising with soda solution. 

By adding a little Dutch pink more orange shades of the 
Fernambuco lake are obtained. In this manner any inter- 
mediate shade between pure red and pure yellow can be 
produced. It is important to lay down a standard series of 
shades, and always to make pigments which exactly corre- 
spond with one of them. It is not possible to make a deter- 
mined shade of red wood lake or any other lake obtained from 
a decoction by working with accurately weighed quantities. 
In general the result will be near the desired shade, but only 



RED WOOD LAKES. 387 

by chance will it be hit exactly. This is because different 
samples of the dye-wood do not always contain the same 
quantity of colouring matter. It is indeed possible with 
practice to estimate approximately from the colour of the 
decoction the shade of the lake which will be obtained from 
it ; but this can only be done exactly by comparing the shade 
of a small batch of the lake with the standard shades. Thus 
in the case of a Fernambuco lake it will be seen by comparison 
with the standard shades whether the colour of the small 
batch is -too deep, too violet, etc. ; then by using more alum 
or more tin solution the defects can be removed. 



CHAPTEK LIX. 

THE COLOURING MATTERS OF SANDALWOOD AND OTHER 
DYE-WOODS. 

IN addition to the dye-woods in more common use, the 
tropics furnish a number of woods which contain fine 
colouring matters, but which are only in restricted use, either 
on account of their cost or because they are not always 
to be bought. To these belong the camwood or barwood 
of Madagascar, the East Indian saiidalwood, and other 
woods which have not yet been used for colouring pur- 
poses even in the tropics. 

The most commonly used of the rarer dye-woods is 
sandalwood, obtained from Pterocarpus santalinus. It is 
garnet red in colour, sinks in water, and produces a fine 
red meal, which rapidly becomes brown in air owing to 
oxidation of the colouring matter. According to recent 
researches, sandalwood contains two colouring matters, one 
obtained from the other by oxidation. Only one of these, 
santalin, is known with certainty. Sandalwood contains 
about 16 per cent, of santalin, which when pure forms 
a crystalline powder, differing from other wood colour- 
ing matters in being almost insoluble in water. Up to the 
present it has only been used in dyeing to obtain handsome 
and fast red colours. From it very bright lakes may be 
obtained, which may be used equally as well as the red 
wood lakes.. The aluminium and tin lakes are especially 
fine. The extract of sandalwood is obtained by boiling 



THE COLOUKING MATTEES OF SANDALWOOD. 389 

very fine chips with water, then adding alcohol and leaving 
the mixture. When stannic chloride or alum, or a mixture 
of the two, is added to the solution of the colouring matter 
a red precipitate is produced which, after washing and dry- 
ing, furnishes a bright red lake. 



CHAPTER LX. 

BLUE LAKES. 

Indigo. The colouring matter of the indigo plant is 
distinguished by great resistance to the action of light, 
air, and chemical reagents. It is thus often cited as a 
particular example of a fast and durable colour. It is 
very largely used in dyeing, and in certain combinations 
is important as a painters' colour. 

Indigo comes into commerce in a great variety of 
qualities, all imported from the tropics, in which alone 
the plant grows. The principal varieties are Indian, 
Java, Egyptian, American and African, but in these mer- 
chants make numerous subdivisions which are often to be 
distinguished from one another only by very small differ- 
ences. Good indigo, whatever its source, has the following 
characteristics : It appears in almost cubical lumps, but 
may also form irregular , masses. The lumps must show a 
small specific gravity, a higher might be caused by inten- 
tional additions of sand or earth. It is without taste and 
smell, and the colour of the finest varieties is not particularly 
bright, but when rubbed with a hard substance, or even 
by the pressure of the finger nail, it acquires a peculiar 
coppery lustre, which is the more intense the purer the 
indigo, and is thus regarded as a commou test of quality. 
Indigo is completely insoluble in almost all solvents ; in 
hot oils a small quantity dissolves, which again separates 



BLUE LAKES. 391 

on cooling. The following are the principal signs by which 
the quality of indigo is recognised in commerce : It forms a 
dull-coloured, loose, light mass of earthy nature, a fresh 
fracture of which has a completely unilorm appearance. It 
must be so porous that it at once adheres to the tongue. 
Uniformity of colour, light weight and fine fracture are the 
most important of the practical tests for good indigo. The 
composition of indigo is well known ; we shall glance rapidly 
over the most important substances which occur in indigo 
in varying quantity. 

The Constituents of Indigo, Indigo contains the follow- 
ing compounds : Indigo blue, indigo red, indigo brown, indigo 
gluten, water and salts. The single important constituent 
for the purposes of the colour maker is indigo blue, or indi- 
go tin. 

Pure indigo blue can be obtained from indigo only by 
a lengthy process. The indigo is boiled in turn with dilute 
sulphuric acid, caustic potash, and strong alcohol. The 
gluten dissolves in the acid, the indigo brown in the potash 
solution, and the indigo red in the alcohol. By these 
operations indigo blue is finally obtained in a state of almost 
complete purity, it now contains only a small quantity of 
inorganic substances. The quantity of pure indigo blue 
obiained in this manner varies, according to the purity of 
the indigo employed, from 20 to 75 per cent. Indigo blue 
can also be obtained by direct sublimation from indigo, but 
the yield is very small, since a large proportion is decom- 
posed by dry distillation. 

The most important constituent of indigo, indigo blue, can 
be obtained in quite pure form by a peculiar chemical process. 
When finely powdered indigo is brought in contact with a 
reducing agent in the presence of a strong alkali, the indigo 
blue is converted into indigo white, which forms a soluble 
compound with the alkali, which compound is stable only so 



392 MINEKAL AND LAKE PIGMENTS. 

long as it is protected from the action of the air. In contact 
with air indigo white is at once oxidised, and is reconverted 
into indigo blue, which separates from the liquid as a soft 
deep blue powder ; after washing and drying it is quite pure. 
Whilst sublimed indigo blue forms needles with a splendid 
coppery lustre, when precipitated from a solution of indigo 
white by the action of oxygen it is obtained as an amorphous 
powder, which acquires the coppery lustre only when rubbed. 

The process of the reduction of indigo blue by a reducing 
agent in the presence of a strong base is not only of theoretical 
interest, but is largely used in practice. Fabrics are generally 
dyed with indigo by immersing them in a solution of indigo 
white (known as the indigo vat), and then exposing them to 
the air. The oxidation of the indigo white then takes place 
on the fibre, upon which the indigo is deposited in a very 
finely divided form ; the blue colour is not immediately 
formed, there is an intermediate green stage. 

Indigo is used principally in dyeing. Certain of its com- 
pounds are used as pigments ; the starting point in the 
preparation of these is a compound of indigo blue with 
sulphuric acid. When indigo is treated with sulphuric acid, 
according to the duration of the reaction and the temperature 
at which it is performed, either the monosulphonic acid or 
disulphonic acid is formed. The reactions proceed according 
to the following equations : 

C 16 H 10 N 2 O 2 + H 2 S0 4 = H 2 + C lti H 9 N 2 O 2 . HSO 3 
C 16 H 10 N 2 O 2 + 2H 2 SO 4 = 2H 2 O + C 16 H 8 N 2 O 2 . (HS0 3 ) 2 

When indigo is treated with eight times its weight of 
strong sulphuric acid, and the solution considerably diluted, 
indigotin monosulphonic acid separates as a fine blue pre- 
cipitate, which dissolves with difficulty in water and alcohol, 
and is insoluble in dilute acids. In order to obtain the 
disulphonic acid 1 part of indigo is digested with 15 parts 



BLUE LAKES. 393 

of sulphuric acid at 50 to 60 C. for several days. If fuming 
sulphuric acid be used the solution is effected in a shorter 
time. The solution is diluted with a large quantity of water, 
when a little monosulphonic acid separates ; the clear solution 
is drawn off and a piece of wool immersed in it. The sulphonic 
acid is soon fixed by the animal fibre ; the wool is washed with 
water and then treated with a solution of ammonium carbonate, 
when a deep blue solution of ammonium indigotin disulphonate 
is produced. This solution is evaporated at a low temperature 
and treated with strong alcohol, which leaves a residue of the 
pure disulphonate, which is dissolved in water, the solution 
precipitated with lead acetate and the lead indigotin disul- 
phonate treated with sulphuretted hydrogen. The colourless 
solution becomes blue on evaporation ; finally pure indigotin 
disulphonic acid is left as an amorphous deliquescent mass. 
The solution became colourless owing to the reducing action 
of the sulphuretted hydrogen ; when this was driven off by 
heating the blue colour reappeared. 



CHAPTEE LXI. 

INDIGO CARMINE. 

INDIGO carmine is used in the laundry, for blue inks and as 'an 
artists' pigment ; in chemical composition it is a mixture of 
the sodium salts of the two indigotin sulphonic acids described 
in the last chapter. In working on the large scale it is par- 
ticularly important that the indigo should be finely powdered 
and completely free from moisture. It is a laborious process 
to powder indigo in mortars, and loss always occurs through 
the formation of dust. Thus when large quantities of indigo 
are to be powdered it is advisable to use rotating cylinders. 
These are strong casks bound with iron hoops and provided 
with a well-fitting slide, through which the indigo and a 
number of iron balls are introduced. When these casks are 
rotated for a sufficient length of time the indigo is converted 
into the finest powder without any loss. 

Indigo Mills, Special mills have been designed for powder- 
ing indigo. They may be advantageously employed for other 
materials also. The construction of a well-designed colour 
mill is represented in Fig. 36, in which indigo may be very 
finely powdered without loss. It consists of a cast-iron pan, 
which can be closed by a well-fitting cover. 'I hrough the 
cover passes the axis of the rotating part, which is moved by 
the bevel cog-wheels and the handle. Above the cog-wheel 
fastened to the axis is a cross piece carrying heavy balls at 
the ends, which serve to keep the motion regular. In the 



INDIGO CARMINE. 395 

interior of the pan four vertical rods are attached to the axis ; 
these are placed at such a distance that they drive before 
them the heavy balls in the circular depression of the pan. 
The action of this apparatus is very simple. After the intro- 
duction of the indigo through an opening in the cover the 
axis is put in rotation, the vertical rods attached to the axis 
roll the balls before them, and thus break up the lumps of 
indigo. The larger pieces of indigo oppose considerable 
resistance to the rotation, which should at first be slow. 
When the lumps have been broken down the speed of rota- 
tion can be considerably increased. After the operation has 




FIG. 36. 

been continued for a sufficient length of time the indigo is 
converted into an impalpable powder. 

To facilitate the removal of the ground indigo, an arm 
carrying a biush of fine hair should be attached to the 
horizontal arm of the axis. If this brush is placed so that it 
sweeps the bottom of the circular depression in which the 
balls move, all the indigo powder in this depression will be 
carried in one direction. In the depression is an opening 
closed by a slide during the grinding. The brush expels the 
ground indigo through this opening when the axis is slowly 
rotated ; it is collected in a vessel below. The larger and 



396 MINERAL AND LAKE PIGMENTS. 

heavier are the balls in this mill, the more rapidly will a 
given quantity of indigo be powdered, and the greater will 
be the power required to drive the machine. By means of a 
pulley on the axis of the cog-wheel the mill can be driven by 
mechanical power. 

The powdered indigo is exposed for a long time to a 
temperature of 120 C., to completely dry it. For this 
purpose it is spread out on sheets of tin plate in a thin layer 
and heated so long as a test portion decreases in weight. 
The powder whilst still warm is at once mixed with the 
sulphuric acid ; 4*5 kilogrammes of fuming sulphuric acid 
and 1 kilogramme of ordinary sulphuric acid (66 Be) are 
used to 1 kilogramme of indigo. Heat is developed in the 
operation, and in order to prevent the indigo from being charred 
the vessel in which the mixture is made is placed in a larger 
vessel filled with cold water. The mixture is stirred with a 
glass spatula until it is quite uniform. The vessel is then 
maintained at a temperature of about 50 C. (not more than 
60 C.) for 7 to 8 days. After the lapse of this time the indigo 
is completely dissolved, and the vessel is found to contain two 
distinct layers, the upper fluid and the lower a paste. The 
contents are mixed with cold water, about 10 kilogrammes 
of water to 1 kilogramme of indigo. A solution of 10 kilo- 
grammes of common salt is then added in small quantities at 
a time. Indigo carmine is insoluble in strong salt solutions; 
it separates as a deep blue precipitate, which is allowed to 
settle and then filtered. If it were attempted to wash out 
the remainder of the salt solution retained by the carmine, a 
considerable quantity of the latter would dissolve as soon as 
the salt solution became diluted to a certain extent. To 
avoid such loss the precipitate is allowed to drain completely 
on the filter, and then spread out upon bricks, which absorb 
the water, so that the carmine forms a stiff paste. 

Indigo carmine, from which the salt has not been com- 



INDIGO CAEMINE. 397 

pletely washed out, dries completely when exposed to the air. 
This may be prevented by the addition of glycerine, which 
is a very hygroscopic substance, and so keeps the carmine 
always moist, thus preventing the crystallisation of the salt. 

Indigo carmine easily dissolves in water to a deep blue 
liquid. It has tremendous colouring power, in which respect 
it is only equalled by genuine cochineal carmine. As men- 
tioned above, indigo carmine is extensively employed in the 
manufacture of ink. The so-called alizarin inks show a very 
pale colour as they flow from the pen, and only become deep 
black after some time. To make these inks equally visible 
at the first indigo carmine is added, by which they acquire a 
handsome blue colour. 

Indigo carmine may also be made by digesting 1 part oi 
dry powdered indigo for '24 to 36 hours with 4 parts of strong 
sulphuric acid, diluting with water, quickly filtering through 
a linen cloth, and adding 4 parts of common salt. The 
precipitate is then collected and dried on gypsum or clay 
plates. . 

Blue Lake, When alum is added to a solution of indigo 
sulphonic acid, and then soda solution, a blue precipitate is 
formed, which, when dry, resembles Chinese blue in appear- 
ance. It is used in painting. This indigo blue lake is 
superior to other blue lakes by reason of its fastness to light. 
It is rather dear, and is mixed with starch, so that it may be 
sold at low prices ; the mixture is formed into slabs and 
sold as " new blue," " indigo extract," etc. It is chiefly used 
in the laundry. 

Under various names a number of blue pigments are sold 
consisting of ordinary qualities of indigo mixed with Prussian 
blue, smalts, chalk, etc. These are used in the laundry, and 
also for distempering. 



CHAPTEE LXII. 

THE COLOURING MATTER OF LOGWOOD. 

THE wood of Hcematoxylon campechianum (logwood), a native 
of South America, comes into the market in small logs of a 
red colour ; it contains a colouring matter whose properties 
are fairly well known. Before the discovery of the coal-tar 
dyes logwood was a most important colouring material ; by 
means of it red. blue, vi<>let a::d black colours can be obtained. 
At present it still plays an important part in dyeing. 

Many varieties of logwood come into commerce. The 
most important are Campeachy wood, Honduras, Jamaica 
and St. Domingo logwood ; the first named is the best, and 
the last the poorest quality. To facilitate the extraction of 
the colouring matter by water logwood is frequently sold in 
chips. Many dye-wood grinders moisten logwood with lime- 
water; this gives the powder a better colour, but diminishes 
the yield of colouring matter. Since the only useful con- 
stituent of logwood can be extracted by treatment w T ith 
water, and the wood is only a carrier of the colouring matter, 
which is only useful for fuel when exhausted, logwood 
extracts are largely used in place of the wood. The extracts 
are black resin-like masses, which easily dissolve in water ; 
they are very hygroscopic, and should therefore be kept in 
closed vessels. 

Logwood Extract Solid extracts can be obtained from 
the majority of dye-woods. The use of these is a great con- 



THE COLOURING MATTER OF LOGWOOD. 399 

venience to the dyer and the colour maker ; the method by 
which extracts are made is therefore briefly described. 

Dye-wood extracts can be made on the small scale, by 
washing out the finely divided wood until soluble matters are 
no longer taken up by the water, and cautiously evaporating 
the united extracts. When the extract becomes concentrated 
the greatest care must be taken to prevent burning on the 
bottom of the vessel ; a burnt extract is always darkened by 
the presence of products of decomposition, and its solution 
has a brown colour. Apart from this, the complete extraction 
of the colouring matter requires much time, and such dilute 
solutions are produced that they cannot be evaporated, but 
at the best can 'be used in the place of water to extract fresh 
quantities of wood. 

When steam is utilised to extract dye-woods these defects 
are removed. A burnt extract is not to be feared, and a small 
quantity of water is sufficient to extract the colouring 
matter completely. The apparatus illustrated in Fig. 37 is 
very suitable for the extraction of dye-woods, and in general 
for obtaining vegetable extracts. The extraction vessel is 
pear-shaped, its two hollow axes move in hollow bearings. 
Thus it may be turned upside down, and steam and water 
can be introduced into the interior through the axes, which 
are connected with the pipes, E, W, and R. The opening 
at the pointed end of the pear is closed by the screw, S ; 
through it the substances to be extracted are introduced. 
In the lower portion of the vessel is a sieve upon which the 
materials are spread out. The opening of the pipe con- 
nected with E and W is below the sieve. 

The process is commenced by introducing the materials 
through the upper opening, fastening the cover down steam- 
tight by the lever, B, and the screw, S, and then running in 
water through W until it flows out through the lower of the 
two narrow pipes which are shown somewhat paler in the 



400 



MINEEAL AND LAKE PIGMENTS. 



shaded portion of the drawing. All the taps are then closed 
with the exception of that on the upper of the two small 
pipes. By opening the cock on R steam is led in. As a 
rule, steam at a low pressure is used, not more than half 
an atmosphere. The contents begin to boil in 15 to 30 
minutes, steam then issues from the open tap. According 
to the nature of the material to be extracted, boiling is con- 
tinued from 40 to 60 minutes. The side tap is then closed, 




FIG. 37. 

and the tap on the pipe E opened. The pressure of the 
steam now forces the liquid up the pipe E ; the pressure of 
half an atmosphere is sufficient to raise it about 4 metres. 
In this way it can be forced into a tank above. The rest 
of the liquid can be run off through the cock, h, at the 
bottom of the vessel. When quite empty the cover is 
taken off, the vessel turned over, and the solid residue 
removed. One extraction of dye-woods is not sufficient to 



THE COLOURING MATTEE OF LOGWOOD. 



401 



remove the whole of the colouring matter; in most cases 
the wood is treated a second time before the apparatus 
is emptied. Even then some quantity of colouring matter 
remains in the wood. The twice extracted wood is brought 
into a tub filled with water, and the solution resulting from 
the long contact of the water with the wood is used in 
the next operation to extract new quantities of wood instead 
of fresh water. The dimensions of the extraction vessel 
vary according to the size of the works. A large apparatus 




FIG. 38. 

costs not much more than a smaller, since in both cases the 
labour is the same, so that it is advisable to use a large 
apparatus. When the copper extraction vessel is made to 
have its greatest diameter about 1 metre, it can be charged 
with about 50 kilogrammes of rasped wood at once. 

The extraction apparatus of Hanig and O. Eeinhard is 
shown in Fig. 38. It can be turned over by means of cog- 
wheels. In the cover are an air valve, e, and safety valve, /; 

below the cover is a coil, k, fed with cold water from c. In 

26 



402 



MINERAL AND LAKE PIGMENTS. 



the extraction, by opening the valve v, steam enters through 
a at I ; it is condensed by the cooled cover, the condensed 
water flows through the material on the sieve, and collects 
in the space B. After some time v and z are opened, the 
liquid is then boiled by the steam in the coil, s. Steam rises 
through the material, is condensed on the cover, and again 
drops down. At the end of the operation the solution is drawn 
off through the cock, h. 




FIG. 39. 

Kohlrausch's Process for Obtaining Concentrated Extracts 
of Colouring Matters and Tannins. When dye-wood or tan- 
bark is continually brought in contact with fresh quantities 
of water, after some time it is exhausted ; if the solution con- 
taining a certain quantity of the soluble matters is brought 
in contact with fresh material not yet extracted, it takes up 
more soluble matter, and thus becomes more concentrated. 



THE COLOURING MATTER OF LOGWOOD. 



403 



The substances contained in tan-bark are very soluble in 
water, so that by the appropriate treatment of a certain 
quantity of bark, divided amongst different vessels, with a 
certain quantity of water, on the one hand the bark can be 
exhausted completely, and on the other very concentrated 
solutions (" bark extracts ") obtained. The process patented 
by Kohlrausch is based upon the principles just stated ; it 
can be used to obtain tannin extracts from tan-bark and 
colour extracts from dye-wood. In this process the raw 
materials need not be finely ground in order to be completely 
extracted ; they may be used in large pieces. It will be 




FIG. 40. 

understood from the following description that fine bark meal 
or finely-rasped dye-wood could not be worked. 

The apparatus consists of a number of 10 to 20 extractors 
connected together, also with a water tank above, and with 
a boiler. A single extractor is represented in section in Fig. 
39, together with the necessary pipes and valves by which it 
is connected with the neighbouring extractors, the water tank 
and the boiler. Fig. 40 is a plan of three extractors connected 
together. 

The extractors consist of wooden or copper vessels, 
slightly conical in shape, and of sufficient strength to resist 



404 MINERAL AND LAKE PIGMENTS. 

the pressure of one atmosphere. On the top is a copper 
dome closed by a lid, through which the raw material is in- 
troduced, and also steam and water. Immediately above the 
bottom is an opening, also closed by a screw, which serves to 
remove the exhausted materials. An inclined sieve is placed 
at some distance above the bottom ; upon this the material 
rests. The extract collects below the sieve, and may be run 
off by the pipe or brought into another extractor. All the 
metallic portions of the apparatus which come in contact with 
the liquid must be made of a metal such as copper, which does 
not act upon tannic acid. Iron cannot be used ; it forms 
deep bluish or greenish black compounds with tannic acid, 
which would cause the extract, instead of being pale and 
clear, to resemble ordinary writing ink. 

In a range of 10 extractors the process is carried out in 
the following manner : The extractors numbered 1 to 10 are 
filled with bark or dye-wood and closed ; 1 is then filled with 
water from the tank, and heated by steam to 50 to 70 C. 
After some time the contents of 1 are forced into 2, and 1 is 
again filled with water, so that the material in 2 is in contact 
with the solution from 1, whilst the material in 1 is warmed 
with a fresh quantity of water under pressure ; the extract 
in 2 is transferred to 3, that in 1 to 2, and 1 is again filled 
with water, and so on. Finally from 10 a very strong extract 
of tannin or colouring matter is obtained. The quantity of 
water required to fill one extractor has come ten times in 
contact with fresh bark or dye-wood. The material in 1 has 
been treated with ten times the quantity of water ; it is now 
exhausted, and is replaced by fresh material. The sequence 
of the vessels is now changed. The original extractor 2 is 
to be regarded as 1, and 1 as 10. After ten repetitions the 
original order of the extractors re-obtains. Ninety-nine per 
cent, of the tannic acid of bark is extracted in this way. 

The concentrated extracts obtained in this apparatus 



THE COLOURING MATTEE OF LOGWOOD. 405 

should be mixed with a little carbolic acid to prevent de- 
composition ; they may then be filled into barrels. The 
extracts may also be so far concentrated by evaporation 
that they become syrupy. The tannins are readily decom- 
posed ; they would be considerably altered if their solutions 
were evaporated in open vessels. The extracts are therefore 
evaporated at a very low temperature under diminished 
pressure in vacuum paiis, which are now much used for 
the concentration of solutions of substances, such as sugar, 
which would be injured by heating above a certain tem- 
perature. Essentially, a vacuum pan is a thiok-walled copper 
vessel, in which the liquid is warmed by a steam coil. It 
is connected with an air pump, which exhausts the air at 
the commencement of an operation, and afterwards steam. 
The liquid is thus constantly evaporated under a low pres- 
sure. Extracts of tan-bark and dye-woods boil briskly under 
these conditions at temperatures below 60 C., at which no 
decomposition of the tannin or the colouring matter is to 
be feared. When the solutions have been evaporated to 
the proper strength they are run off directly into the pack- 
ages in which they are to be despatched, and in which they 
become syrupy or even solid masses, according to the extent 
to which the evaporation has been driven. 

The packages should be at once closed ; the thick extract 
is thereby most simply and safely prevented from decom- 
posing, to which risk it would be exposed by the access 
of mould spores. If these were already present in the 
barrels or were communicated to the extract by the air, 
they would either be killed by the hot liquid or would be 
prevented from developing for a long time. In the closed 
vessels the extracts remain completely unaltered. 

The concentrated solution of colouring matter obtained 
by extracting dye-woods in the above apparatus can be at 
once used, after dilution, in the preparation of lakes and in 



406 MINEEAL AND LAKE PIGMENTS. 

dyeing, but it is not an extract in the ordinary commercial 
sense of the term, i.e., it does not solidify on cooling. To 
obtain solid extracts the concentrated solutions must be 
evaporated ; concentration with fire heat would be attended 
with danger to the quality of the extract, steam heat is 
therefore used. The liquid to be evaporated is brought into 
shallow steam-jacketed pans, in which the operation is con- 
tinued until the liquid solidifies into a resin-like mass when 
dropped on cold stone. When sufficiently evaporated the 
extract is allowed to solidify, broken into lumps, and these 
packed whilst still warm into barrels lined with paper. The 
lining is necessary on account of the hygroscopic nature of 
the extracts ; when they are exposed to the air they absorb 
water and form a viscous fluid which soon becomes mouldy. 
A properly prepared dye-wood extract should dissolve in 
water without residue, and the solution when largely diluted 
should show the characteristic colour of the wood with no 
brownish shade, and when the colouring matter is precipitated 
from such a solution by a metallic salt the residual solution 
should be almost colourless. If the extract dissolves incom- 
pletely in water and the solution is brown after precipitation 
of the colouring matter, the* extract has been burnt in the 
evaporation. 



Logwood and logwood extract contain two substances 
of importance in dyeing and colour making. These are 
hsematoxylin and hsematem. Haematoxylin is found in 
logwood in greatest amount shortly before the wood is 
cut. When pure it forms colourless crystals of a peculiar 
sweet taste, which are soluble in cold water with difficulty, 
more easily in hot, and readily soluble in alcohol or ether. 
The composition of hsematoxylin is expressed by the formula 
C I8 H 14 C . 

Hsematoxylin is not a colouring matter. It is important 



THE COLOURING MATTER OF LOGWOOD. 407 

because from it is obtained the essential colouring matter of 
logwood haematein. When a trace of ammonia is added to 
the colourless solution of haematoxylin, the liquid at once 
becomes dark red owing to the formation of haemateiu. 
When a larger quantity of ammonia is added the liquid 
acquires a deep red colour, and then contains only haematem 
(its ammonia compound), which is formed according to the 
following equation : 

C 16 H 14 O 6 + NH 3 + = C 16 H 9 O 6 . NH 4 + 2H 8 0. 

To obtain haematein in the pure state it is then only 
necessary to add sufficient acetic acid to decompose the 
haematein ammonia compound. Haematein separates as dark 
violet crystals, which readily dissolve in water and alcohol ; 
its aqueous solution gives blue precipitates with the majority 
of the metallic salts. This behaviour of haematoxylin towards 
ammonia explains the increase in colouring power of log- 
wood, which has been exposed for a long time to the action 
of the air in the rasped state. Through the action of the 
ammonia in the air a larger quantity of the haematein 
ammonia compound has been formed. It has been proposed 
to facilitate the formation of this ammonia compound by 
moistening rasped logwood with a very dilute glue solution 
and allowing it to lie in the air. This process can only have 
the object of utilising in the formation of haematein the 
ammonia resulting from the decomposition of the glue, but in 
this decomposition deep-seated reactions occur, which might 
affect the haematein itself. It thus appears more suitable to 
effect the formation of haematein by the direct use of ammonia. 
This can be done with little cost by watering heaps of rasped 
logwood with ammonia and repeatedly shovelling about the 
wood so that it comes into contact with the air. The author 
has found that the conversion of haematoxylin to haematein is 
very complete in this process ; care should be taken not to 



408 MINERAL AND LAKE PIGMENTS. 

make the layer of rasped wood too deep, and to take its tem- 
perature frequently. In the transformation of hsematoxylin 
to hsemateiin the temperature rises, the rise might be injurious 
if it proceeded too far. Thus if the temperature of the 
interior of the heap is found to be high the wood should 
be turned over. 

The solution of the logwood colouring matter produces 
handsome lakes, all of which have, however, the inconvenient 
property of acquiring an ugly grey colour on long standing. 
The finest and most durable of the logwood lakes is known as 
violet lake, which is made with alumina salts. The best result 
is obtained when a solution of aluminium acetate, obtained 
by precipitating alum with lead acetate, is mixed with a 
logwood decoction or a solution of logwood extract. The 
precipitate is pale or deep violet according to the amount 
of aluminium salt added. After drying to a certain point at 
a gentle heat it can be mixed with gum solution to a paste, 
which is then completely dried. 

Logwood is most valuable in dyeing and calico printing, 
in which it serves to produce a fine and durable black. When 
potassium chromate is added to a decoction of logwood a deep 
black liquid results, which can be used as a good and cheap 
writing ink. If somewhat stronger solutions are used a 
greenish precipitate first separates, which soon acquires a 
pure black hue ; it is the chromium lake of hsemateiin. This 
compound is very durable and is largely used in dyeing to 
produce fast blacks. The black precipitate might be dried 
and used as an artists' black pigment if the carbon blacks 
were not cheaper and more durable. 



CHAPTEK LXIIL 

GREEN LAKES. 

THE yellow colouring matters produce green compounds 
with copper salts. These are important because they can 
be produced without great cost, and are not as poisonous 
as the arsenic pigments, although copper compounds are 
far more poisonous substances than should be used for cer- 
tain purposes. Their employment might be attended with 
danger if used to colour children's toys and pictures. 

These green copper lakes are simply made by adding 
copper sulphate solution, free from iron, to a hot decoction of 
yellow berries or weld until the liquid is emerald green, and 
then adding caustic soda solution in small quantities to pre- 
cipitate the lake. The temperature of the liquid should not 
be more than 50 to 60 C. when the caustic soda is added. 
It has been maintained that these lakes turn out well only 
when the precipitation is so conducted that the residual 
liquid is quite colourless. The author has, however, found 
it more satisfactory to discontinue the addition of caustic 
soda when the liquid is somewhat coloured, since when the 
precipitation is complete other substances besides colouring 
matter may be precipitated. 

Chlorophyll, All the higher plants contain the same 
green colouring matter chlorophyll. It has a very fine 
shade, is fairly fast, and the raw material from which it 



410 MINEEAL AND LAKE PIGMENTS. 

is produced is to be obtained at nominal cost. Special 
attention should, therefore, be paid to this colouring 
matter. 

Chlorophyll is very easily obtained. Grass of a good 
green, green leaves, or any green portion of a plant is 
allowed to stand with weak caustic soda solution in a 
large vessel for twenty-four to thirty hours. The liquid is 
poured off, brought to the boil for a moment, at once filtered 
and neutralised with hydrochloric acid. The chlorophyll, 
which has been dissolved by the caustic soda, is then thrown 
down as a grass green precipitate ; after washing and drying 
it can be used as a pigment. A chlorophyll lake can be 
obtained by dissolving the colouring matter in caustic soda 
and adding alum solution. The green precipitate is a com- 
pound of chlorophyll and alumina. 

Tschirch gives the following method for obtaining chloro- 
phyll : Green leaves (grass), not containing tannin, are 
extracted with boiling alcohol, the extract cooled, filtered 
and evaporated until a sticky mass remains. This is washed 
with hot water until the washings are colourless, and the 
residue extracted with cold alcohol. The solution is evapor- 
ated to half its volume, the separated crystals are dissolved 
in alcohol, and the solution heated on the water bath with 
zinc dust. A deep emerald green solution with a red 
fluorescence is obtained ; it can be preserved unaltered for 
a long time in blue glass bottles. It retains its colour in 
diffused daylight also for a lengthy period. A beautiful 
green chlorophyll lake is obtained by boiling this solution 
with the solution of an aluminium salt and precipitating 
with soda. Unfortunately, the lake is not fast. The 
chlorophyll solution obtained by this process is quite in- 
nocuous, and is well adapted for colouring liqueurs and 
confectionery. 

Sap Green, This lake, also known as bladder green, is 



GBEEN LAKES. 411 

obtained from unripe yellow (Persian) berries. It can be used 
as a water colour, but not in oil painting. This pigment differs 
from the ordinary yellow berry lake in colour and chemical 
composition ; the yellow lake is a compound of xanthor- 
hamnin with the metallic oxide, whilst sap green contains 
an uncrystallisable bitter substance, rhamnocathartin. 

The lake is obtained from yellow berries which are not 
quite ripe. They are broken up and left in a warm place. 
The mass soon ferments ; after about ten days it is pressed. 
Four parts of the liquid are then mixed with 0'5 part of 
alum and 0*5 part of potassium carbonate. The salts are 
dissolved in boiling water, and the solutions added to the 
hot sap. The mixture is then evaporated to the consistency 
of a thick syrup, which is generally packed in animal bladders, 
hence the name of " bladder green ". Some care is necessary 
in the evaporation to prevent the burning of the soft mass 
on the bottom of the vessel. If the evaporation is carried 
somewhat further the mass solidifies on cooling ; it is then 
black, and only transmits green light at the edges. The 
use of potash in preparing sap green is attended with the 
disadvantage that the hygroscopic nature of the salt prevents 
the colour from drying. If magnesia is used instead of 
potash the lake dries far more rapidly, but has much less 
covering power. Sap green is principally used for colouring 
paper and leather. 

Orange-coloured lakes also can be obtained from yellow 
berries by precipitating the decoction with stannic chloride. 
These lakes are not directly employed, but are produced 
immediately upon the fabric in dyeing. 

Chinese Green, Lokao, Under this description a green 
lake has recently been imported from China. It is a valuable 
pigment. Chinese green is sold in flat cakes, which are blue, 
with a green or violet lustre. The powdered lake is pure 
green and partially soluble in water ; it contains colouring 



412 MINEEAL AND LAKE PIGMENTS. 

matter, water and inorganic matter consisting chiefly of clay 
and lime. 

Chinese green is made in China in a peculiar manner 
from the twigs of certain species of Bhamnus. According to 
report, the bark is boiled with water and a cotton cloth 
immersed in the decoction. The fibre fixes a colourless 
substance which becomes green on exposure to air. The 
cotton is repeatedly dipped into the decoction until it has 
absorbed a large quantity of colouring matter. It is then 
washed with cold water and boiled with water upon which 
cotton yarn is laid. The colouring matter suspended in the 
liquid adheres to the yarn, which is washed with a little cold 
water, and the colouring matter then collected upon paper 
and dried. 

If Chinese green is really made by this process, it is not a 
lake but a vegetable colouring matter, to which clay has 
been added to increase its weight or to make it plastic. 
Very fine lakes can be obtained from Chinese green by 
dissolving it in a solution of alum, and adding soda solution. 
When Chinese green is dissolved in acetic acid and ammo- 
nium chloride and a zinc salt added, a blue lake is produced 
on the further addition of sodium acetate. A bluish violet 
lake is obtained by treating Chinese green with a strong 
reducing agent and adding calcium acetate. 

Charvin's Green, The very high price at which genuine 
Chinese green was sold whilst it was still a novelty, gave 
rise to attempts at imitation. Charvin, of Lyons, succeeded 
in producing a pigment from Rliamnus which has the char- 
acteristic property of Chinese green of retaining its colour 
in artificial light. He plunged the bark of common buck- 
thorn into boiling water, boiled for a few minutes, and 
allowed the whole to stand for twenty-four hours. Lime- 
water was added to the brown liquid, which was then exposed 
to the action of the air in shallow vessels, when it gradually 



GEE EN LAKES. 413 

turned green and deposited a green precipitate. When this 
appeared, the liquid was brought into glass vessels and 
potassium carbonate solution added so long as a pre- 
cipitate resulted, which after drying had all the properties of 
genuine Chinese green. 

Although for some time Chinese green was a very fashion- 
able colour, its use is now almost discontinued, although it 
can be used with advantage as an artists' pigment, and its 
preparation by Charvin's method is neither difficult nor 
specially costly. 



CHAPTEK LXIV. 

BROWN ORGANIC PIGMENTS. 

Asphaltum is found in large deposits in several regions of 
the earth. It is a compound of carbon and hydrogen, and 
varies in appearance from cobblers' wax to tar. Asphaltum 
is of organic origin and is closely related to the mineral oils. 
Among the pigments usually employed by artists asphaltum 
plays an important part ; it produces very warm shades be- 
tween brown and deep black. The preparation of asphaltum 
for artists' use is very simple. Good uniform lumps, free 
from sand and other impurities, are coarsely powdered and 
mixed with a solvent in a well-stoppered flask. Asphaltum 
readily dissolves in essential oils, and also, though with more 
difficulty, in fatty oils. It dissolves slowly at the ordinary 
temperature, but quickly on warming. On account of the 
inflammability of the essential oils generally used, certain 
precautions must be taken. The asphaltum is mixed with 
turpentine in a large flask which is well closed and heated 
by hot water. In a short time a viscous mass is formed 
which is mixed with the solvent by shaking. 

Asphaltum may also be prepared, without dissolving, by 
powdering and grinding with oil exactly as a mineral pig- 
ment. 

Sepia. This brown pigment has a warm hue which is 
not readily surpassed. It is an animal product ; the cuttle- 
fish, a species of cephalopod which abounds in all warm 
seas, produces a peculiar colouring matter which is stored in 
a sac, commonly called the ink-bag. It is used by the 
animal for protection, when pursued it ejects the contents 



BROWN ORGANIC PIGMENTS. 415 

of the bag, and thus makes the surrounding water so dark 
that it is enabled to escape. 

Sepia is made almost exclusively in Italy. The contents 
of the ink-bag are quickly dried, and then rubbed with strong 
caustic lye to a thick pulp. More caustic lye is then added, 
and the mixture heated almost to boiling ; from the filtered 
solution sulphuric acid precipitates the pure colouring matter. 

As one of the most handsome brown pigments, sepia is 
largely used, but unfortunately it can only be used as a water 
colour. On account of its cost, sepia is frequently imitated. 
Vegetable substances are charred, extracted, and the extract 
concentrated until it solidifies on cooling ; it is then finely 
powdered, made into a paste with gum Arabic or tragacanth, 
and formed into cakes. All such imitations are, however, so 
imperfect that they are at once recognised. On comparison 
with genuine sepia, none of them is found to possess the 
warm shade peculiar to sepia. 

Brown colouring matters can be obtained by heating all 
soft portions of plants. The products have a deep brown 
shade in consequence of the high proportion of carbon they 
contain. When the young twigs of soft woods are exposed 
in closed cylinders to a temperature of about 300 to 400 C., 
and the residue powdered, colours ranging from a dark rust 
brown to almost pure black are produced. The higher the 
temperature employed the more nearly the shade approaches 
to black. 

Parts of plants which contain sugar or similar compounds 
become deep brown at comparatively a low temperature. The 
colour, which varies from brown to hyacinth red, is due to 
caramel. This substance is produced when coffee, beetroot, 
or chicory root, all of which contain a large proportion of 
sugar, are heated. Such pigments are little used ; [they 
cannot be ground with oil, and in water can rarely be mixed 
with other colours. 



CHAPTEK LXV. 

SAP COLOURS. 

UNDEE the designation of " sap colours " several pigments are 
brought into the market in the condition in which they are 
ready for immediate use in printing. The term is practically 
restricted to lake pigments which form a transparent layer 
when dry, thus the sap colours may be defined as dissolved 
lakes mixed in a viscous medium, such as thick gum solution. 
In general the sap colours are not much used, since when 
wetted with water they again dissolve, which is not the case 
w r ith pigments ground in oil. Yet for certain purposes they 
are commonly used, as in the manufacture of playing cards. 

In the manufacture of these pigments only colouring 
matters soluble in water can be used, the number of which 
is restricted. According to a particular method a tin lake 
is first made, which is decomposed by a strong base, such 
as ammonia, so that the colouring matter again goes into 
solution. The very deep coloured liquid is then mixed with 
a thickener and an indifferent white substance such as flour 
or starch, formed into thin sticks, and sent into the market. 

A decoction of buckthorn berries is used for the yellow 
sap colours. It is considerably evaporated, and then mixed 
with 2 to 3 per cent, of alum, after which starch paste or 
gum Arabic and sugar are added in proper quantity, and 
the mixture evaporated at a low temperature so that it is 
not browned by over-heating. 



SAP COLOUES. 417 

When a tin lake is employed in the preparation of a 
sap colour, the precipitate obtained by adding stannic chloride 
to the solution of the colouring matter is washed, and, 
without drying, treated with a small quantity of strong 
ammonia ; generally about 10 per cent, of the volume of 
the precipitate is sufficient to dissolve the lake. It is always 
better to leave a portion of the precipitate undissolved than 
to modify the colour by the addition of too much ammonia. 
The precipitate should be dissolved in a glass vessel ; the 
liquid is well stirred, covered over, and allowed to stand 
until the undissolved portion of the precipitate has settled. 
The ammoniacal solution of the colouring matter cannot be 
concentrated by evaporation. Thus, before the precipitate 
is dissolved it must be well freed from water by long draining. 
To give the solution the proper consistency about an equal 
quantity of thick gum solution is added, and then sufficient 
starch to form a paste which can be rolled into thin sticks, 
which are then dried upon boards at a gentle heat. Sap 
colours are made in exactly the same way from any other 
colouring matter adapted to the purpose. In using these 
colours it is simply necessary to bring them into water ; if 
they contain only gum and sugar in addition to colouring 
matter they completely dissolve, but if they contain starch 
the solution is incomplete. As a rule starch is only added 
to give the colour the consistency requisite for printing. 

Red sap colours can be made either from red wood lake 
or from cochineal carmine ; the former is used in ammoniacal 
solution, the latter as a solution of the pure carmine in 
ammonia. The colour is prepared from red wood by allow- 
ing a decoction to stand for several days, adding 2 to 3 
per cent, of alum, evaporating and thickening with gum 
solution. This colour is cheap, but cannot compare in 
beauty with that obtained by treating red wood tin lake 

with ammonia. The solution of genuine carmine in am- 

27 



418 MINEEAL AND LAKE PIGMENTS. 

monia produces a sap colour which leaves nothing to be 
desired in regard to shade, but on account of the cost of 
carmine it can but rarely be used. A solution of pure 
indigo carmine mixed with gum solution is used as a blue 
sap colour ; it can be shaded by additions of red or yellow 
colours. Chinese blue may be used instead of indigo 
carmine ; it dissolves in a solution of oxalic acid. When 
Chinese blue, which has been thoroughly washed, is mixed 
whilst still moist with a little saturated oxalic acid solution, 
the blue dissolves, and the solution may then be made into a 
paste by the addition of thickening materials. 

Green sap colours are made, as a rule, by mixing yellow 
and blue. All shades can be thus obtained. A green colour 
of different composition is obtained by boiling the violet 
solution of chrome alum with gum solution until the colour 
changes to green. Violet colours are obtained by mixing 
red and blue. Sepia produces a brown sap colour. 

It has been proposed to use glucose in place of the much 
dearer gum Arabic, and also to use malt extract as thickener. 
These substances absorb water from the air, and thus the 
colours prepared with them never become quite hard, but 
always remain pasty. For some purposes this is a decided 
advantage, since such colours can be readily taken up by 
the brush and easily rubbed up with water, but they cannot 
be formed into cakes as is usually done with water colours. 
When the colours are to be made into cakes they must be 
thickened with gum Arabic alone, without glucose or malt 
extract. The cakes must be dried with great care ; if they 
are dried too quickly they will crack or even fall to pieces. 



CHAPTEE LXVL 

WATER COLOURS. 

THE colours thus called are so prepared that they easily 
mix with water to such a condition that they can be applied 
with the brush. Special pigments are not required for this 
purpose, ordinary dry colours are simply mixed with a binding 
medium soluble in water, and the paste is generally pressed 
into cakes. In commerce very different qualities of water 
colours are found, ranging from cheap children's playthings 
to the most costly colours used by painters in water colour. 
Those pigments which are not already in very fine powder, in 
consequence of the process by which they are made, such as 
chrome yellow, must be subjected to a very careful process of 
levigation. To simplify as far as possible this laborious pro- 
cess, it should be conducted by grinding the pigment as finely 
as possible, well stirring it in a tub with water, allowing to 
rest for several minutes, and then running off the liquid into 
a second tub, from which it is again drawn off into a third 
after several minutes. The liquid remains in the last vessel 
until it is quite clear, when the deposit is collected. This is 
now so fine a powder that a powerful microscope is required 
to distinguish the separate particles. The residue in the 
first and second vessels consists of the coarser particles; it 
is again ground with fresh material. In this process regard 
must be given to the specific gravity of the material : the 
higher it is the shorter is the time during which the liquid 



420 MINEEAL AND LAKE PIGMENTS. 

is allowed to remain, in the tubs, since the coarser particles 
of heavy substances settle very rapidly. In dealing with the 
light lakes, such as the alumina lakes, the liquid in which 
the lake is suspended must remain much longer at rest. 
Two vessels may be used instead of three, without danger of 
the precipitate containing coarse particles. 

Gum Arabic and tragacanth are used as binding materials 
for the pigments. Dextrine is also much used in place of 
the expensive gums ; for this purpose only pure white dextrine 
should be used, since the brown colour of ordinary dextrine 
would injure the shade of the colours, especially of pale 
colours. Gum Arabic and dextrine, which are readily soluble 
in water, require no special preparation before their solutions 
are mixed with the pigments. The solutions are made by treat- 
ing with water ; they are then allowed to stand for several days 
in a tall vessel so that impurities may settle. If the solutions 
are very turbid, they are filtered through closely woven linen. 

Gum tragacanth requires rather different treatment. It 
is not completely soluble in water, in which it only swells up 
to a great extent. It is prepared by leaving it for several 
days in water, and when it has swollen, rubbing the slimy 
mass in a mortar until it is completely uniform. 

The levigated pigments are allowed to dry in the air to a 
soft paste, which is mixed with the proper quantity of gum 
Arabic and tragacanth solutions. As a rule the two gums 
are used together. The colouring matter and binding medium 
may simply be ground together, but long grinding would be 
required to produce a completely homogeneous mixture. 
The costly manual labour is therefore as far as possible 
replaced by machinery, by which means a cheaper and also 
more uniform product is obtained. The machines by which 
the pigment is ground with the binding material are of 
simple construction. They consist of rollers placed in pairs 
one above the other and moving in opposite directions. The 



WATER COLOURS. 421 

two rollers of each pair are connected by cog-wheels, in such 
a way that the lower moves more slowly than the upper. In 
consequence of this arrangement, the rollers, in addition to 
crushing, exert a grinding action upon the viscous mass 
passing between them. As a rule, after the colour has 
passed through one pair of rollers, it goes through a second 
and third, from the last of which it is taken off by a scraper. 
It is now completely uniform. 

The binding medium must be of such consistency that, 
after grinding with the pigment, a fairly stiff paste results 
which is suitable for pressing. The cakes of colour are 
pressed out in an ordinary spindle press, which should be so 
constructed that the stamp in the down stroke comes upon 
the paste beneath it- and stamps out cakes upon which the 
engraving of the die is clearly shown. If the cakes crack 
when slowly dried, the medium contains too much gum 
Arabic. If the impression of the die upon the cakes is not 
sharp, and if they remain rather elastic when completely dry, 
too much tragacanth has been used. It is rather difficult to 
grind a stiff paste, many manufacturers prefer to grind the 
colour in a rather more fluid condition, and then to allow the 
paste to thicken by drying to the consistency necessary for 
the production of good cakes. 

The moulds in which the cakes are made must be very 
carefully worked in metal, so that the description and trade 
mark print clearly upon the cakes. The cakes should be 
dried at the ordinary temperature or but little higher. They 
are placed upon smooth boards, and care is taken that the 
temperature of the drying-room remains uniform, which is 
the condition requisite for the production of the fewest cracks. 
When it is intended to place on the market only faultless 
and well-stamped cakes they must be sorted when dry, and 
the cracked ones rejected ; these can be worked up in the next 
operation. 



422 MINEEAL AND LAKE PIGMENTS. 

The dry cakes are given a good appearance by coating 
them with a weak solution of gum, and then drying. Ac- 
cording to the price at which these colours are to be sold 
the cakes are given a different character. The finest colours 
are generally made into larger cakes and packed in hand- 
some boxes, whilst ordinary cheap colours are made into 
small lumps or circular plates, flat on one side and some- 
what convex on the other, and packed in boxes of soft 
wood. 

Moist Water Colours, Instead of grinding water colours 
with gum Arabic or tragacanth and bringing them on the 
market in the dry state, they may also be sold in a condition 
resembling that of oil paints. This may be accomplished by 
using very viscid glucose syrup instead of gum Arabic, and 
grinding the pigments in this exactly as in oil. Glucose is 
very hygroscopic, the colours prepared with it remain moist, 
and may be spread out upon the palette like oil colours. It 
is then only necessary to wet the brush in water and mix the 
mass with it in order to obtain colour of the proper consistency. 

Moist water colours have also been known as "honey 
colours," since this mixture of sugars was formerly used for 
their preparation ; it also is hygroscopic. Honey is no longer 
used ; the much cheaper glucose answers the same purpose. 
On account of their semi-fluid nature, moist water colours are 
put up in tubes just as oil colours. They are thus rather dear, 
but are little used. 



CHAPTEK LXVIL 

CRAYONS. 

CEAYONS are coloured pencils by which pictures may be, 
so to speak, " dry painted ". At present this method of 
painting is little used, though it was in vogue in the last 
century, but coloured pencils, especially blue and red, are 
much used for writing. Crayons are now used in a very 
convenient form, being generally made in the same manner 
as lead pencils. The coloured mixture from which the 
crayon is made is produced in the form of a paste con- 
taining a soft, finely-ground mineral, well mixed with the 
colouring matter and an amount of binding medium just 
sufficient to hold the powders together. 

Gypsum is generally used as the soft white mineral 
forming the base of the coloured mass. It is much better 
to use steatite (soap-stone), which is not much dearer, and 
has many advantages over gypsum. The preference given 
to steatite is based upon a comparison of the properties of 
the two minerals ; gypsum is crystalline, steatite is non- 
crystalline. Powdered gypsum has essentially a dry char- 
acter, whilst powdered soap-stone is of a peculiar greasy 
nature, and consequently can be readily smeared upon a 
flat surface ; it also imparts a pleasing lustre to the colours 
with which it is mixed. 

The manufacture of crayons consists in preparing and 
moulding the coloured mass. The process is commenced 



424 MINEKAL AND LAKE PIGMENTS. 

by mixing the powdered colour and soap-stone in a closed 
rotating cylinder. The rotation is continued until the 
mixture is uniform in colour. The quantity of colour to 
be mixed with the soap-stone depends upon the shade to 
be produced. Manufacturers who make a speciality of 
crayons prepare a number of shades of each colour. The 
artists who use crayons, however, usually require only those 
of a pure shade, since they themselves can produce the inter- 
mediate shades from the essential colours. On this account 
it is advisable to make crayons of distinct and deep colours. 
The mineral pigments are the most suitable : for yellow, 
deep chrome yellow ; for red, vermilion or deep madder 
lake ; for green, one of the pure green pigments, such as 
chrome green ; for blue, Chinese blue or ultramarine ; for 
brown, burnt sienna or manganese bistre. For white and 
black crayons levigated chalk is used, alone or mixed with 
a sufficient quantity of fine vine black or some other good 
black. Either gum Arabic or size may be used as binding 
material. When the former is employed the crayon becomes 
so brittle when dry that it breaks when pointed with the knife , 
in spite of the greatest care. Size produces a less brittle 
crayon, and is also much cheaper, so that it is preferred. 

The paste from which the crayons are to be formed is 
made by mixing thin size with the colouring matter and 
steatite to a soft pulp, which is then kneaded to secure 
uniformity. The process then proceeds in a different manner 
according as th3 crayon is to be pressed or sawn out. 

For producing the crayons by pressing, a simple apparatus 
is required. It consists of a horizontal metal cylinder with a 
well-fitting piston. The front of the cylinder is closed by a 
metal plate in which is an orifice of rather greater diameter 
than the crayons to be made. In front of the cylinder is 
an endless band which moves away from it. In using this 
apparatus to shape the crayons, the paste is made of such 



CRAYONS. 425 

consistency that a slight pressure forces it out of the orifice 
in the front of the cylinder in a coherent rod. When the 
cylinder is filled with the paste, care must be taken that 
it includes no air bubbles, for these would cause the rod to 
break. The piston is then put in position and the mass 
forced by uniform gentle pressure out of the narrow opening. 
The endless band must move away at the same rate as the 
rod proceeds from the cylinder, so that a long stick of the 
crayon mass rests upon it. This stick is cut up by a blunt 
knife into uniform lengths, which are dried upon boards 
covered with blotting paper, and are then enclosed in casings 
similar to those of the ordinary lead pencil. The rods must 
be given a rather greater diameter than they are to possess 
when dry, since they shrink somewhat in drying. 

The second method consists in producing a thick paste 
which is moulded into blocks of the length of the crayons. 
These blocks are very slowly dried at a uniform temperature, 
and when completely dry, are cut by a fine saw into thin 
rods, which are then enclosed in a wooden case. The powder 
produced in sawing is used in the next operation. This 
simple process has many drawbacks. In the first place it is 
difficult in drying large blocks of the crayon mass to avoid 
cracks, which must be carefully filled with thin paste. Also 
in sawing up the blocks a large number of rods will be 
broken, even with the most careful treatment (these are 
again worked up with the powder produced in sawing). 
Thus, generally, the formation of crayons by pressure is 
preferred. 

It is important to dry the crayon rods so far that they do 
not shrink further after they are placed in the case ; other- 
wise they would break up when sharpened. At times it is 
quite impossible to make a usable point on such a faulty 
crayon. 

Crayons for Earthenware, Crayons for this purpose are 



426 MINERAL AND LAKE PIGMENTS. 

made, according to M. Hosier, by finely powdering colours 
suitable for glass and earthenware, and mixing them to a 
paste with a solution of 2 parts of gum Arabic and 1 part of 
Marseilles soap. Pencils are then moulded from the paste 
and wrapped in a strip of paper just previously dipped in 
a paste of gypsum. Upon unglazed articles or ground glass 
these crayons can be used like a lead pencil. The colour is 
fixed by burning in the usual manner. 



CHAPTEE LXVIII. 

CONFECTIONERY COLOURS. 

CONFECTIONERS employ several colours. Yellow, brown 
and black are usually obtained by means of caramel ; the 
other colours, red, green and blue, which are also largely 
used, must be made from harmless compounds. Unfor- 
tunately, colour makers have been known to offer the 
consumer for this purpose colours which by no means 
correspond to this requirement ; even the poisonous arsenic 
pigments have been used for colouring sweets. In such a 
case the user of the colours is less to be blamed than the 
manufacturer, who should sell for this purpose only colours 
in no way injurious to health. Fortunately we possess 
among the pigments of organic origin a sufficient number 
to satisfy these requirements : for green, either sap green or 
leaf green may be used without hesitation ; for red, cochineal 
carmine is well suited, and for blue, indigo carmine. The 
two pigments last named are indeed expensive, but since 
they possess great colouring power they can still be used for 
colouring cheap sweets. 

For colouring liqueurs equal care must be taken to choose 
non-injurious colours. Those mentioned above may also 
be used for this purpose. Kecently these colours have been 
largely displaced by the aniline dyes, which are particularly 
adapted by their beauty and great colouring power for the 
colouring of confectionery or liqueurs. There should be 



428 MINERAL AND LAKE PIGMENTS. 

some hesitation in using these dyes for colouring articles 
of food. Some of them are made by means of arsenic 
compounds, and it is very difficult to free them absolutely 
from every trace of arsenic. But supposing the dye to be 
free from arsenic, there may still be objections to its use, for 
the pure dyes themselves may possess poisonous properties, 
and thus ought not to be used for colouring articles of food. 

For colouring foods the colours above mentioned are 
sufficient. Orange is obtained by mixing caramel with red, 
violet by mixing red and blue, and the shades thus produced 
are quite sufficient for the purpose in question. The colour 
maker should sell the colours in such a condition that they 
can be used without further preparation. The colours for 
confectioners and liqueur makers should be put on the 
market in a semi-fluid or pasty condition. For this pur- 
pose carmine should be ground with a very thick sugar 
syrup. Leaf green and indigo carmine require no further 
preparation ; they are already semi-fluid, they readily diffuse, 
and are also soluble in alcoholic liquids. 



CHAPTEK LXIX. 

THE PREPARATION OF PIGMENTS FOR PAINTING. 

ACCORDING to the purpose for which they are to be used 
pigments require a different method of preparation. The 
preparation of pigments for certain special purposes, such as 
sap colours, cake colours and water colours, has been already 
described. The present chapter deals with the preparation 
of those pigments which are used in large quantities by 
artists and for ordinary painting. 

Colours for artistic purposes require different treatment 
to those used for ordinary painting. It is important that 
both should be ground to a completely homogeneous mixture 
with the binding medium. At first sight this appears to 
be a simple operation, but in practice there are difficulties 
which are not too easily overcome. Artists' colours are 
generally ground with a drying oil. The drying oils are 
vegetable oils which, when exposed to the air in a thin layer, 
in a short time become very viscous, and finally completely 
resinify; linseed, poppy, and nut oils possess this property. 
As a rule, artists' oil paints are ground with poppy oil. To 
prevent the paint from drying to a solid mass by exposure 
to air it is enclosed in protecting vessels. Formerly the 
ground paint was sold in small bags of bladder. These are 
now no longer used ; instead, collapsible tubes made of a soft 
tin alloy are employed. They are closed at one end ; the 



430 MINERAL AND LAKE PIGMENTS. 

other has a neck upon which a metal cap screws down. 
Pigments ground with just sufficient oil to form a thick 
paste may be preserved in these air-tight tubes without 
altering in consistency. 

Cheaper pigments, such as white lead, white zinc, chrome 
yellow, etc., which are principally used in ordinary painting, 
are ground with raw or boiled linseed oil. Ordinary boiled 
oil is made by boiling linseed oil with litharge ; it contains a 
certain quantity of lead in solution. It has been repeatedly 
stated in this work that lead compounds are very sensitive 
to sulphuretted hydrogen. Pigments which are not them- 
selves altered by sulphuretted hydrogen acquire a dark shade 
when they are ground with ordinary lead-boiled oil, because the 
lead is slowly but certainly converted into black lead sulphide 
on exposure to air. Thus, in order to retain the original 
beauty of colour, lead-boiled oil should be replaced by oil 
boiled with manganese borate, which is at least as cheap and 
has the advantage that the oil does not darken in air. Now 
that zinc oxide is so cheap it is more and more used in 
place of white lead, it is not sensitive to sulphuretted 
hydrogen, and does not even change its colour in an atmo- 
sphere of the pure gas. It seems to be quite illogical to 
use this pigment with a lead-boiled oil. A coating would 
be quite discoloured in the course of time, whilst, if used 
with manganese-boiled oil, it would retain its white shade 
unaltered. 

The dry pigment was formerly mixed with the oil by 
manual labour. The pigment was spread out upon a smooth 
stone slab, the oil poured over, and the two substances rubbed 
to a uniform paste by means of a glass or stone rubber known 
as a muller. 

Paint Mills are constructed in various ways. The mix- 
ture of colour and oil is ground either between two metal 
plates or between two rollers pressed close together. 



THE PREPARATION OF PIGMENTS FOB PAINTING. 431 

Fig. 41 represents the construction of a paint mill in 
which the grinding is accomplished by a rotating disc. The 
mixture of pigment and oil stirred together is brought into 
the hopper, T, from which it must pass between the rotating 
grinding disc, M, and the lower surface of the hopper. The 
two substances are thus mixed together. The ground paint 
flows out of the ring-shaped vessel surrounding M into a 
receiver below. The grinding disc is driven by means of 
horizontal and vertical toothed wheels, the latter of which is 




FIG. 41. 

connected with a pulley driven by power. By means of a 
screw on the lower part of the plate in which is the bearing 
of the axle of the grinding disc, the distance of the latter from 
the lower surface of the hopper can be adjusted. The process 
begins by adjusting the disc at a considerable distance from 
the hopper. When the paint has once gone through the 
apparatus the disc is raised so that the colour is more finely 



432 MINEEAL AND LAKE PIGMENTS. 

ground. The ground paint is returned to the hopper until 
it is quite uniform. 

This form of mill can be made in different sizes. Fig. 42 
shows the construction of a form for grinding by hand. The 
actual grinding arrangements are exactly the same as those 
previously described ; the difference lies simply in the use of 
a fly-wheel turned by a handle. The illustrations are due to 
the kindness of W. Sattler of Schweinfurt, who makes paint 
mills as a speciality in different sizes and of excellent quality. 

In the second form of paint mill the paint is passed 




FIG. 42. 

between smooth rollers moving in opposite directions with 
different speeds, which thus exert a grinding action in addition 
to the crushing effect. The two rollers are provided with 
cog-wheels with a different number of cogs in order to give 
the different speeds. 

In order that the paint may be sufficiently ground in one 
operation, roller mills are made with several pairs of rollers, 
one above the other, the lower rollers being somewhat nearer 
together than the upper. The paint is fed on to the top pair 
of rollers, and, after going through these, passes to the next, 



THE PREPARATION OF PIGMENTS FOR PAINTING. 433 

and finally, after going between all the pairs of rollers, collects 
in a receiver below in a finished condition. By the use of 
such mills the paints can be ground sufficiently fine in one 
operation, if a proper number of rollers is used. When only 
two or three pairs are used and the paint is to be very finely 
ground, it must be passed through the whole mill two or 
three times. 



28 



CHAPTEE LXX. 

THE EXAMINATION OF PIGMENTS. 

WHEN an accurate examination of a pigment is required, 
the only course is to conduct an exact chemical analysis, 
which can only be done by an expert chemist in a well- 
equipped laboratory. But often it is necessary in the course 
of trade to decide rapidly the nature of a pigment or to 
detect the adulteration of a dear pigment with a cheaper, 
and for this purpose it is important to have a method of 
examination which can be conducted without much ap- 
paratus, and which requires no great chemical knowledge. 
It is quite possible to test the majority of pigments in a 
simple manner. Few reagents are necessary. For an ex- 
amination of the mineral pigments the following are generally 
sufficient : hydrochloric, nitric and sulphuric acids, caustic 
soda solution and ammonium sulphide. 

The examination of a pigment containing organic com- 
pounds is somewhat more difficult, especially when it is 
necessary to ascertain the nature of the colouring matter. 
In this case additional reagents are required stannous 
chloride, alum solution, etc. 

Since the present work is intended to meet the require- 
ments of the practical man, the behaviour of the different 
pigments towards ordinary reagents is given in tabular 
form, the pigments of the same colour being taken together. 
Since the colouring matters of organic origin, as they occur 



THE EXAMINATION OF PIGMENTS. 435 

in the lakes, require a rather more detailed examination, the 
testing of pigments composed of inorganic materials only is 
first given, afterwards the properties of the organic colouring 
matters, so far as is necessary, will be added. 

Mineral Pigments. If the substance to be examined is in 
the form of a dry powder, it may at once be tested with the 
reagents mentioned, but if it is a paint or water colour, the 
oil or gum must be first removed, otherwise it would not be 
possible to recognise with certainty the action of the reagents. 

From water colours, which are ground with gum Arabic 
or tragacanth solution, it is easy to separate the pigment 
from the binding medium. The colour is allowed to stand 
in a tall narrow beaker with a somewhat large quantity of 
water. After some time the lumps become soft, they are 
then repeatedly stirred up, the pigment allowed to deposit, 
and the water, which now contains the binding medium, 
poured off. 

The removal of oil from a paint is somewhat more diffi- 
cult. A quantity of the paint is placed in a flask with a 
mixture of equal parts of strong alcohol and ether, or with 
benzine. The flask is lightly corked, and, after frequent 
shaking, allowed to stand. The liquids mentioned are good 
solvents for oils. After a day or two the pigment will 
generally have deposited at the bottom of the flask. The 
solution is then poured off, the residue mixed with a small 
quantity of the solvent and transferred to a filter, where 
another small quantity of solvent is poured over it. The 
solvent is allowed to drain off, and the residue is dried. A 
powder without coherence should be left ; this is the pig- 
ment free from oil. It can be treated like an originally 
dry colour. 

The pigments are most conveniently examined in test 
tubes. If these are not at hand the reactions may be carried 
out on a plate of glass lying on white paper. The powder 



436 MINEKAL AND LAKE PIGMENTS. 

is placed on the glass and the reagent dropped on to it 
from a glass rod dipped in the liquid. 

Examination with the Blowpipe. From the behaviour 
of pigments at high temperatures important conclusions 
may be drawn as to their nature. For this purpose small 
porcelain crucibles are used ; broken pieces of porcelain or 
an iron spoon may also be employed. An ordinary spirit 
lamp is generally sufficient as a source of heat. In some 
cases a higher temperature is required, which is obtained 
by means of the blowpipe. 

The blowpipe is an invaluable instrument in the examina- 
tion of mineral pigments. By means of it, almost without 
reagents, the nature of the pigment can generally be ascer- 
tained. The reagents necessary in using the blowpipe are 
soda, borax, and the solution of a cobalt salt. 

The following method should be observed in examining 
pigments by means of the blowpipe. A quantity of the sub- 
stance, about equal in volume to two grains of rice, is placed 
in a small hole cut by a knife in a piece of charcoal, where 
it is heated by the blowpipe flame. The behaviour of sub- 
stances is different in the oxidising and reducing flames 
of the blowpipe. When a flame is blown out to a point 
by means of the blowpipe it may be seen that the flame 
consists of two conical portions, one inside the other. The 
inner is known as the reducing flame, because metallic 
oxides heated in it produce a bead of metal, or, as the change 
is chemically expressed, are reduced to metal. The outer 
cone has the opposite properties ; metals melted in it are 
quickly changed into oxides by the action of the oxygen of 
the air, which has unrestricted access. In examining a 
pigment with the blowpipe the reducing flame is first 
used. The nature of the bead of metal, such as is readily 
obtained from lead pigments, often allows the composition 
of the substance to be recognised with certainty. If the 



THE EXAMINATION OF PIGMENTS. 437 

behaviour of the bead of metal is not conclusive it is further 
heated in the oxidising flame. The metal is thus converted 
into oxide, which deposits on the charcoal, and by its colour 
and volatility or want of volatility enables the metal con- 
tained in the pigment to be determined. 

A solution of cobalt nitrate or chloride is used in testing 
for certain metallic oxides. The substances are moistened 
with a very dilute solution of one of these substances before 
heating. After they have been heated they show character- 
istic colours if certain oxides are present. 

Several metallic oxides give a characteristic colour when 
fused with borax. For this purpose a small loop is made 
at the end of a thin platinum wire ; this is moistened and 
dipped iu powdered anhydrous borax. On heating in the 
blowpipe flame the borax adhering to the wire melts to 
a colourless glass. In testing a pigment the transparent 
bead of borax is then dipped into the powder, and again 
fused in the blowpipe flame. It is of great importance in this 
test to fuse but a very small quantity of material in the borax 
bead ; some metallic oxides have such great colouring power 
that, when too much is used, the bead appears quite black, 
so that its colour cannot be recognised. 

REACTIONS OF THE WHITE PIGMENTS. 

On Heating on Charcoal : Lead pigments give a lead bead 
in the reducing flame, which is converted in the oxidising 
flame to lead oxide, forming a deposit on the charcoal 
surrounding the hole. 

Antimony white gives a brittle metallic bead in the 
reducing flame. This burns in the oxidising flame with the 
evolution of white vapours, and is at the same time covered 
by small shining crystals. 

Bismuth white in the oxidising flame gives a rainbow- 
coloured incrustation spreading far over the charcoal. 



438 



MINERAL AND LAKE PIGMENTS. 



Tin white gives a malleable metallic bead, converted by 
nitric acid into a white powder. 

Zinc white is converted into a green mass when moistened 
with cobalt solution and heated in the oxidising flame. 

The more expensive white pigments are frequently mixed 
with cheaper substances, e.g., white lead with finely powdered 
barytes, chalk or gypsum. Such admixture is distinctly to 
be regarded as adulteration, since the added substances have 
not the covering power of white lead. In the case of coloured 
pigments an addition of a white material is not to be re- 
garded as adulteration, since the addition is made in order to 
impart a paler shade to the colour. 



Pigment. 


Hydrochloric Acid. 


Caustic Soda, 


Ammonium 
Sulphide. 


On Heating 


Antimony white . 


Dissolves, solu- 


Dissolves. 


Turns reddish 


Turns yellow 




tion turbid on 




yellow. 


and melts. 




adding water. 








White lead . . 


Dissolves with 


Dissolves. 


Turns black. 


Becomes per- 




effervescence, 






manently 




solution gives 






yellow. 




crystalline 










lead chloride. 








Lead oxychloride 


Dissolves with- 


Dissolves on 


Turns black. 


Turns yellow. 




out efferves- 


boiling. 








cence. 








Lead sulphate . 


Insoluble. 


Dissolves on 


Turns black. 


Unaltered. 






boiling. 






Permanent white 


Insoluble. 


Insoluble. 


Unaltered. 


Unaltered. 


(barium sulphate) 








Bismuth white . 


Insoluble. 


Insoluble. 


Turns black. 


Evolves reddish 








brown fumes, 








which redden 








litmus paper. 


Zinc white . . 


Dissolves with- 


Dissolves. 


Unaltered. 


Turns yellow, 




out efferves- 






becomes white 




cence. 






again on cool- 










ing- 


Tin white . . . 


Dissolves. 


Dissolves. 


Turns yellow. 


Unaltered. 



THE EXAMINATION OF PIGMENTS. 
KEACTIONS OF THE YELLOW PIGMENTS. 



439 



Pigment. 


Hydrochloric Acid. 


Caustic Soda. 


Ammonium 
Sulphide. 


On Heating. 


Chrome yellow 
and chrome red 


Green solution 
above a white 
residue, which 
is soluble on 
largely dilut- 
ing. 


Becomes or- 
ange on boil- 
ing and dis- 
solves. 


Blackened. 


Fuses to a yel- 
low mass. 


Cassel yellow . . 


Unaltered, be- 
comes white 
on boiling. 


Becomes paler 
on boiling, 
liquid yel- 
low. 


Blackened. 


Melts. 


Naples yellow 


On boiling or- 
ange, then 
white. 


Reddish yel- 
low. 


Turns brown- 
ish black. 


Melts at a high 
temperature. 


Massicot . . . 


Turns white. 


Partially sol- 
uble on boil- 
ing. 


Blackened. 


Melts with 
some difficulty. 


Lead iodide . . 


Turns white. 


Dissolves. 


Blackened. 


Melts. 


Barium yellow 
(chromate) 


Yellow solution 
which gives 
white precipi- 
tate with sul- 
phuric acid. 


Unaltered. 


Unaltered. 


Unaltered. 


Cadmium yellow 


Dissolves with 
evolution of 
sulphuretted 
hydrogen. 


Unaltered. 


Unaltered. 


Melts with diffi- 
culty. 


Zinc yellow . . 


Yellow solution. 


Yellow solu- 
tion, white 
residue. 


Unaltered. 


Melts with diffi- 
culty. 


Cobalt yellow . . 


Red solution. 


Colourless sol- 
ution, grey- 
ish blue pre- 
cipitate. 


Unaltered. 


Becomes black- 
ish at high 
temperatures. 


Orpiment . . . 


Unaltered. 


Colourless sol- 
ution. 


Yellow solu- 
tion. 


Volatilises. 


Turpeth mineral 


Dissolves. 


Unaltered. 


Blackened. 


Turns red. 



On Heating on Charcoal : Chrome yellow, chrome red, 
Cassel yellow, massicot and lead iodide give lead beads in the 
reducing flame. Chrome yellow and red when fused with 



440 



MINERAL AND LAKE PIGMENTS. 



soda give red masses soluble in water. Naples yellow gives 
a lead bead and white fumes without smell. 

Orpiment gives an odour of garlic. 

Cobalt yellow, when heated with alumina, is turned blue. 

Cadmium yellow produces a brown incrustation on the 
charcoal. 

The examination before the blowpipe serves especially 
for the recognition of lead, chromium, antimony and arsenic. 
If a pigment blackens on heating, the presence of an organic 
colouring matter is indicated, such as Dutch pink, weld 
lake, etc. 

The adulteration of pigments free from lead by colours 
containing that metal is recognised by the blackening pro- 
duced by ammonium sulphide. 

EEACTIONS OF THE KED PIGMENTS. 



Pigment. 


Hydrochloric Acid. 


Caustic Soda. 


Ammonium 
Sulphide. 


On Heating. 


Chrome red . . 


Green solution, 
white residue 
solub 1 e on 
largely dilut- 
ing. 


Yellow solu- 
tion and 
white resi- 
due. 


Turns green- 
ish black. 


Puses. 


Eed lead . . . 


Chlorine is 
evolved, white 
residue. 


Almost un- 
changed. 


Turns black. 


Turns yellow 
and fi n a 1 1 y 
melts. 


Ferric oxide pig- 
ments 


Slowly dissolve 
to yellow solu- 
tion. 


Unaltered. 


Slowly black- 
ened. 


Become dark 
blackish brown. 


Antimony vermi- 
lion 


Dissolves with 
evolution of 
sulphuretted 
hydrogen. 


Dissolves to 
colour less 
solution. 


Becomes dar- 
ker, partially 
soluble. 


Melts. 


Mercury vermi- 
lion 


Unaltered. 


Turns yellow- 
ish. 


Unaltered. 


Volatilises, sul- 
phur dioxide 
evolved. 


Mercuric iodide . 


Dissolves to col- 
ourless solu- 
tion. 


Dissolves to 
yellowish 
solution. 


Blackened. 


Fuses and then 
volatilises. 


Realgar. . . . 


Unaltered. 


Dissolves to 
colourless 
solution. 


Dissolves to 
yellow solu- 
tion. 


Volatilises. 



THE EXAMINATION OF PIGMENTS. 



441 



On Heating on Charcoal : Chrome red and red lead give 
lead beads in the reducing flame. The former gives a red mass 
when fused with soda, which dissolves to a yellow solution. 

Ferric oxide pigments become darker, but give no in- 
crustation. 

Antimony vermilion burns with production of sulphur 
dioxide and white fumes without smell when heated in the 
oxidising flame ; when fused with soda before the blowpipe, 
it gives a white brittle bead of metallic antimony. 

Vermilion volatilises in the oxidising flame and gives a 
smell of sulphur dioxide. 

Mercuric iodide readily fuses and volatilises. 

Eealgar volatilises. When heated with soda in the reduc- 
ing flame, white fumes with an odour of garlic are produced. 

KEACTIONS OF THE BLUE PIGMENTS. 



Pigment. 


Hydrochloric Acid. 


Caustic Soda. 


Ammonium 
Sulphide. 


On Heating. 


Prussian, Chinese, 
Paris, Turnbull's, 
and Brunswick 
blue 


Dissolve to 
green solution, 
then yellow. 


Decolourised, 
brown resi- 
due. 


Liquid yel- 
lowish green. 


Blackened. 


Mountain blue . 


Dissolves to 
yellowish green 
solution. 


Blackened. 


Blackened. 


Blackened. 


Ultramarine . . 


Rapidly decom- 
posed with 
evolution of 
sulphuretted 
hydrogen. 


Unchanged. 


Unchanged. 


Unchanged. 


Smalts .... 


Almost unal- 
tered, greenish 
solution o n 
long boiling. 


Unchanged. 


Blackened. 


Fuses at a high 
temperature. 


Cobalt blue . . 


Unchanged. 


Unchanged. 


Unchanged. 


Infusible and 
unchanged. 



On Heating on Charcoal : Prussian, Chinese, Paris, Turn- 
bull's and Brunswick blue are turned black, the residue 
colours the borax bead pale brown in the oxidising flame, 
and pale green in the reducing flame. 



442 



MINERAL AND LAKE PIGMENTS. 



Ultramarine is unaltered at a high temperature. 

Smalts, on long heating in the reducing flame with 
borax, gives a dark blue bead. 

Cobalt blue is infusible ; it colours the borax bead blue. 
The bead loses its fine colour on long heating in the reducing 
flame. 

Mountain blue is blackened before the blowpipe. When 
the residue is moistened with hydrochloric acid and again 
heated, the flame is coloured bright green. When fused with 
borax in the oxidising flame, an emerald green bead is 
formed. 

KEACTIONS OF THE GREEN PIGMENTS. 



Pigment. 


Hydrochloric Acid. Caustic Soda. 


A sJuphide. n On Heatin S- 




i 


1 


Verdigris . . . 


Dissolves to Unaltered. Blackened. Blackened with 


(all varieties) 


green.solution, 




' evolution of 




smell of acetic 




peculiar odour. 




acid. 




Bremen green and 


Green solution 


Unaltered. 


Blackened. 


Blackened. 


Brunswick green 


and white resi- 










due. 














Emerald green, 


Dissolve to 


Gradually col- 


Become Blackened and 


Scheele's green 


greenish solu- 


oured brown- 


brownish evolve garlic- 




tion, i ish yellow. 


black, like odour. 


Copper borate 


Dissolves to 


Black residue. 


Becomes Fuses. 




greenish solu- 




brownish 




tion. 




black. 




Rinmann's green 


Dissolves to 


Unaltered. 


Blackened. 


Unaltered. 




rose red solu- 








tion. 






Chromium oxide 


Almost unal- 


Unaltered. 


Becomes dark 


Unaltered. 




tered. 




dirty green. 




Chrome green 


Becomes deeper 


Unaltered. 


Becomes dark 


Unaltered. 


lake 


in colour. 




dirty green. 




Manganese green 


Dissolves to 


Dissolves to 


Is discoloured. 


Unaltered. 




green solution. 


green solu- 










tion. 










Green ultramar- 


Is decolourised 


Unaltered. Unaltered. 


Unaltered. 


ine with evolution 






of sulphuretted 






hydrogen. 






i 





THE EXAMINATION OF PIGMENTS. 



443 



On Heating on Charcoal : Verdigris, Bremen and Bruns- 
wick greens give black residues on charcoal, which produce a 
bluish-green bead when fused with borax in the oxidising 
flame. 

Emerald green and Scheele's green behave in a similar 
manner, but on heating evolve an odour of garlic. 

Hinmann's green gives a blue borax bead. 

Manganese green is discoloured in the reducing flame. 



EEACTIONS OF THE BROWN PIGMENTS. 



Pigment. 
Lead brown . . 


Hydrochloric Add. 


Caustic Soda. 


Atiiraonium 
Sulphide. 


On Heating. 


White residue, 
chlorine evol- 
ved. 


Dissolves. 


Blackened. 


Turns yellow 
and fuses. 


Manganese brown 


Dissolves to 
yellow solu- 
tion. 


Unaltered. 


Becomes flesh 
coloured. 


Unaltered. 


Pyrolusite brown 


Dissolves to 
yellow solu- 
tion, chlorine 
evolved. 


Unaltered. 


Becomes flesh 
coloured. 


Unaltered. 


Prussian brown . 


Dissolves to 
yellow solu- 
tion. 


Unaltered. 


Blackened. 


Turns to reddish 
brown. 


Iron brown . . 


Dissolves to 
yellow solu- 
tion. 


Unaltered. 


Blackened. 


Unaltered. 


Chrome brown . 


Dissolves to 
greenish yel- 
low solution. 


Yellow solu- 
tion, black 
residue. 


Blackened. 


Blackened. 


Cobalt brown 


Dissolves tol Unaltered or 
reddish yellow blackened, 
solution. 


Blackened. 


Unaltered. 


Hatchett brown 


Unaltered. 


Becomes 
greenish blue. 


Blackened. 


Blackened. 


Humins, Bistre . 


Unaltered, yel- 
low liquid. 


Give yellow 
liquid. 


Unaltered. 


Burn. 



444 MINEEAL AND, LAKE PIGMENTS. 

On Heating on Charcoal : Lead brown gives a lead bead 
in the reducing flame. 

Manganese brown and pyrolusite brown, when fused with 
saltpetre on platinum foil at a high temperature, give a 
bluish green mass. 

Prussian brown and iron brown give a pale green borax 
bead in the reducing flame, which turns yellowish brown in 
the oxidising flame. 

Chrome brown heated, moistened with hydrochloric acid, 
and again heated, colours the flame green. It also gives a 
green borax bead. 

Cobalt brown produces a blue borax bead. 

Humin substances burn when heated on charcoal. 

KEACTIONS OF THE BLACK PIGMENTS. 

Almost all black pigments consist of carbon, upon which 
reagents have no action. They should be at once heated 
on charcoal. If they burn away completely in the oxi- 
dising flame they consist of lamp-black or carbon obtained 
by some process of incomplete combustion ; if a white, 
infusible residue is left, the pigment is bone (ivory) black ; 
if the residue is black the substance under examination must 
be " neutral tint," chrome black, or chrome-copper black. 
The two former give a pale green borax bead, whilst chrome- 
copper black gives a deep green bead, and when heated, 
moistened with hydrochloric acid, and again heated, it 
colours the flame green. 



CHAPTEE LXXI. 

EXAMINATION OF LAKES. 

IN examining lakes it is necessary to ascertain the nature 
of the organic colouring matter, the base with which this 
is united, and also the nature of the substances mechani- 
cally mixed with the lake to lighten its shade. 

It is most convenient to examine the pigment first for 
mechanical admixtures. If an effervescence follows the 
addition of hydrochloric acid, denoting the presence of a car- 
bonate, this is generally calcium carbonate. White lead is 
rarely used, since it is very heavy and is more expensive. 
Gypsum and magnesia are occasionally added to lakes ; they 
are recognised by examining the residue left on ignition. 

In testing for the metallic oxide which is united with 
the colouring matter a small quantity of the lake is heated 
in a little porcelain crucible until all organic substances are 
completely destroyed. The residue is then examined for 
aluminium and tin oxides, by means of which lakes are 
most frequently prepared. 

It is not easy to decide which colouring matter is 
united with the metallic oxide, because the organic colour- 
ing matters do not give, as a rule, decided reactions. In 
examining pigments of this kind it is always advisable to 
test at the same time a pigment of known composition, 
and to compare the reactions of the two. The colouring 
matter of the lake is first brought into solution. This is 





s 


N 


d 

a 


& 


"o 


o 


is 




T3 


S 8. _g 


03 




o 




ft 




-S 


^ ^ ^ 






1 


OS 


^^ ^ 


M 


c3 
<X> ^_. 

1 


W. 


1 i 


11 i 


1 

'So 




> S T 3 

*5| 

5P^ 


1 


s 

^ H 

|l 

gft 


^^-3 
Mil 

03 3 

> '" 

3^ * 


|l 


"S 

05 S> 

III 


Ammonia 
oration. 


ft 
^ 

"^ "ol 


1^1 

l|2 

rH ^ ' 

5 ft 










01 . 




<U ' 






-2 







3-2 3 


,-C ^ 


2?" 




C 

11 




U) | 


1 1 


i-t 1-t 


111 


-t 

,. 


1 



s| 



HI 



.ft o 

'o " 

'2 

ft ^ 






M 



Ferric 
liloride. 



Sal 1 



gi-U 

g Sb S 
K 2 



!l 

s o 

II 









II 









03 

^ >> 



^ 

p 



5:& 



o 



a 



,c 

.2 & 
a o 



I II 

& O^ 



ft 

0? 

s 



.S 

"I |l 

r ft1 

p 



CD ? 5i T3 f "3 f j?"S 

2,2 -2 gJ2 ,2 "S I' 



2 a 3 
> > o 

111 

i> ^ O 

i- M 



& S 

_2-2 



sa 8.1 



Ssl 3 



2t? 



ft 

pfl 



3.s 





g-a 
I 


.1 ' 


fi 




| 




1 1 


5 


T- 


! a 1 


|^ 




ji 




.I .1 


S, 


f J i 


1* l" ts 

lg> * ii 

^J '43 ^ ft A g 

fl J* 11 


i i i|l 
|f 


1 

1 

'o 
fl 


1 



1 1 
.- .- 
^ S 


1 ' 

G "CD 




1 








| 




| 


1 


1 5 I 


1 1 1 


1 


i 1 


g 


i 1 


1 || 




> 








1 




1 ^ 










11 


-1 




| 


1 


1 1 1 


1 1 1 


| 


"*!& 1 


1 


1 1 


| I'Si 










o ^ 


1 




*l 




* 






3) 


| 




1 a| 




1 % o'~3 1 


1 1 1 


| 


[o'ft | 


1 


! 1 






^ll 






p- 


|g 




& 




"S .2 


.S 

08 


~c8 


11 


Ii 




II 


1 


1 ^ 1*= 


' '! ' 


ft '^ 


i 4 -3 ' 


11 


1 1 






5 PH ^ 


PH 




PI 


03 -^ 

Si 




PQ 




g 


o5 


^ 


-3 


j ^ 




^ a" 


1 


^2 " 
1 ^ 
J? "3 

5 * 


1 || 1 


^3 

as 

" 



PH 


^^ 

*!' 


ii 


1 1 


1 it! 

o 












2 ^i" 






I 


1 


1 S 1 


I 




r2 ,2 


e 


II || 


Id 

1 II 


0) 




^3 a) 


3 


D 'D 


^ ^ 


& <& u u 


i 1 ^H 




^H ^H 


[> ^ 


^H 


^" ^ 


s S 


PH ^ PH ^ 


^W 












> s 

s 














"S <D 


ea 








1 


S ^ 




gl 


| 


1 si 


.2 s 


' 


1 S c -* 

3 Sj 


^ 1 1 
II 


' 


|t' 


3 


o^ o> 
PQ ^ 


1 ^ o 

PH ^ 


rt 


d 


o 


g 


i 






' -d . 


Yellowi. 
brown 
solutioi 


"S "sc 

111 


i ~ S e - -3 
^1 3-2 1 

O r 

C ^ <u 

> > q5 


3 I 

P, 


1 |l 


I 


11 11 

|6 5| 

53 03 


2^2 1 2 


B 


^ 1 ^* 


1 | 1 


| 


1 1 


"S 


|| S.1 


s' . 


tlnaltt 


| |a 
^ m 


| g^ S 


1 

'S 

PH 


1 1 


g 
g 




^ o "S 
a- 6 ^1 


1 2 


-s 



















c 


* 










M 


.0 

















o carmine 
lake 


q'l 

.' 1 BI 


<D ^5 

K (B 

rw ^ 

o <5 

.2 J ^ 


QJ 

3 


z i 

o o 




' 
w 

g 

Ofe M 


z " 

I: 1 

PQ a3 " 


y 


&JO 

5 ,P 


S o" 

c5 J ^ 


1 


PH P^ 




1 J 


1 1 





448 MINEEAL AND LAKE PIGMENTS. 

accomplished by the action of dilute hydrochloric acid, 
which decomposes the compound of metallic oxide and 
colouring matter, the latter dissolving. A portion of 
the lake should remain undecomposed, so that the solu- 
tion has no strong acid reaction by which the action of 
the reagents to be used would be modified. When the 
lake has been treated for some time with dilute hydrochloric 
acid, water is added and the solution of the colouring matter 
filtered from the residue. Small quantities of this solution 
are then treated with the different reagents. By a com- 
parison of the reactions given by the solution and by a 
solution of the pure colouring matter, the nature of the 
colouring matter under examination may be decided. 

In deciding in the first place if a colour is of organic 
origin, it is treated with hydrochloric acid in the above 
manner. If a coloured solution results, an organic colour 
is probably present. Chlorine water is then added to a 
portion of the solution ; if the latter is quickly decolourised an 
organic colouring matter is certainly present, for they are 
all decomposed by the continued action of chlorine. 

The following reagents are used in testing organic 
colouring matters : dilute sulphuric or hydrochloric acid, 
caustic soda solution or lime water, strong nitric acid, 
which, in consequence of its oxidising properties, gives 
different reactions to the other acids. Of the metallic 
salts, alum, stannous chloride and ferric chloride are used, 
and occasionally copper acetate. Glue solution is also 
used in the pure form of isinglass or gelatine solution, 
with which several colouring matters give characteristic 
precipitates. The foregoing tables give the behaviour of 
the colouring matters contained in the organic pigments 
towards the reagents mentioned above. 



CHAPTEE LXXIL 

THE TESTING OF DYE-WOODS. 

IN the manufacture of pigments from dye-woods or other 
organic substances, the value of the raw material is in pro- 
portion to the amount of colouring -matter it contains, other 
things being equal. It is specially desirable to estimate 
accurately the colouring matter in expensive materials such 
as indigo and cochineal. 

There are a number of methods which permit an accurate 
estimation of the indigo blue in indigo. One good process is 
founded upon the decomposition of indigo blue by chlorine, 
when the colour of the solution changes from blue to yellow. 
Since a definite amount of chlorine is required to decompose 
indigo blue, from the quantity of chlorine required by a 
sample of indigo, its content in indigo blue can be ascer- 
tained. 

Whilst the percentage of indigo blue contained in indigo 
can be found with tolerable accuracy, though by a rather 
elaborate process requiring special apparatus, there is no 
convenient method for examining the other organic colour 
materials by which their content in active constituents can 
be readily found. In practice a process is particularly valu- 
able which requires little time and no complicated apparatus. 
Colouring materials can be [rapidly tested by a physical 
process which requires little time and an inexpensive ap- 
paratus. Under similar conditions the extract of a dye-wood 

29 



450 



MINERAL AND LAKE PIGMENTS. 



is deeper in colour in proportion to the colouring matter it 
contains. If therefore the intensity of the colour of the extract 
can be accurately measured, there is no difficulty in draw- 
ing a certain conclusion as to the amount of colouring matter 
in the raw material. 

The Colorimeter is the apparatus adapted for the purpose 




FIG. 43. 

in question. There are many forms of colorimeter. The 
instrument devised by Dubosq is distinguished by simplicity 
and accuracy of results before other apparatus of similar 
construction. Dubosq's colorimeter consists of the following 
parts (Fig. 43) : two glass cylinders, C and C lt the bottoms of 
which must be perfectly plane both inside and outside (the 
accuracy of the results depends upon this), stand upon a sheet 



THE TESTING OF DYE-WOODS. 451 

of plate glass. Two glass cylinders of smaller diameter, 
T and T\, are suspended in C and C^ The bottoms of these 
cylinders must also be quite plane. It would be very ex- 
pensive to make glass cylinders of this kind in one piece. 
The same result is obtained by providing each cylinder with 
a metal ring upon which screws another ring in which is 
cemented a circular piece of plate glass. 

Light should only reach the eye of the observer in a 
direction parallel with the axis of the cylinders. C and C t 
are therefore blackened on the outside. The inner cylinders, 
T and T 1? are fastened to racks moving vertically. The 
distance through which the cylinder is moved is measured 
by a scale on one of the racks. Above the cylinders T and 
T l are Fresnel's prisms. Below C and C l is a mirror, S, 
which can be adjusted to throw light vertically upwards. 
Beams of light pass through the plate glass and the bottoms 
of the cylinders C and T, C x and T 1? unrefracted, they are 
then deviated by the Fresnel's prisms so that the observer 
looking down through the telescope, F, has a circular field of 
view, one half of which is illuminated by the light passing 
through the cylinder C, and the other by the beam passing 
through C r The intensity of the light which has passed 
through the two cylinders can thus be accurately compared. 

In order to use this apparatus to compare the intensity 
of colour of two liquids, the following process is performed : 
A liquid is made, the colour intensity of which is taken as 
100. The colour intensity of the liquid under examination 
will then be represented by a number indicating the relation 
between its intensity and that of the standard liquid. A 
solution of caramel in water is generally used as the stan- 
dard, since this substance has very great intensity of colour. 
The preparation of absolutely pure caramel is difficult, and 
it is therefore advisable, in order always to have the standard 
solution of the same intensity, to prepare a large quantity 



452 MINEEAL AND LAKE PIGMENTS. 

of caramel solution at once, to add carbolic acid to prevent 
it from decomposing, and to keep it in well-closed bottles. 
When the standard solution is almost used up the colori- 
meter is employed to prepare a fresh quantity of equal 
intensity. 

To render possible an exact comparison of two sub- 
stances they must be tested under the same conditions, 
that is, the solutions of the colouring matters must be 
made in exactly the same manner. Finely-powdered materi- 
als dissolve more readily than coarse powders. In making 
the solutions which are to be examined for intensity of 
colour, the raw materials must be brought into a condition 
of fine division by the same instrument, for example, a rasp. 
The colour solution is made by boiling 100 grammes of the 
dye-wood with exactly a litre of distilled water for precisely 
thirty minutes. The liquid is then filtered into a 1-litre 
flask. The dye-wood absorbs water, and some is lost by 
evaporation, so that considerably less than 1 litre of liquid 
is collected in the flask. Distilled water is added to make 
up the volume to 1 litre. 

If two samples of logwood are treated in this manner, 
solutions are obtained which contain the colouring matter 
in the same proportions in which it exists in the two 
samples. The intensity of colour of the solutions is then 
estimated in the following manner : The cylinder C is filled 
with the standard caramel solution up to a mark on the out- 
side of the cylinder. The cylinder C l is filled with the 
decoction to the same height. The distance between the 
bottoms of the cylinders, K and T, K x and T lt must be made 
smaller in proportion to the intensity of colour of the liquid 
between them. If now the amount of light which penetrates 
a layer of caramel solution of a certain thickness be taken 
ab unity, the depth of a layer of the decoction must be 
greater, the smaller the quantity of colouring matter it 



THE TESTING OF DYE-WOODS. 453 

contains, in order that the two halves of the field of view 
may be equally illuminated. The cylinder T l must be 
raised to a greater height the smaller the quantity of 
colouring matter in the liquid, in order that the two halves 
of the field may be illuminated to the same extent. If 
the colouring power of the caramel solution is taken as 
100, the colouring power of the decoction can be readily 
calculated from the height to which the cylinder, T T , is 
raised. The heights of the layers of liquid between the 
bottoms of the cylinders C and T, C x and T lf are inversely 
as the quantities of colouring matter contained in the re- 
spective cylinders. 

When caramel solution is used as the standard the 
intensity of the light in the two halves of the field can be 
judged, but not the intensity of colour. In order to estimate 
the latter a solution of that colouring matter must be used as 
a standard which is the principal constituent of the decoc- 
tion under examination. Thus, in a careful examination of 
logwood a solution of haematoxylin would be used, and in 
the examination of red wood a solution of pure brasilin, as 
the standard. In this case a saturated solution of the 
colouring matter would be taken as the standard. If the 
intensity of its colour were represented by 100, the colour 
intensity of the wood under examination would always be 
less than 100, and would, with tolerable accuracy, represent 
the percentage of colouring matter in the wood. The result 
would not be quite accurate, because the dye-wood contains 
other substances which dissolve in water on boiling and 
affect the colour of the decoction, but the results are of 
such accuracy that for practical purposes no material mis- 
take will be made by taking them as percentages of colour- 
ing matter. 

Although at first sight the estimation of the colouring 
matter in a dye-wood by means of the colorimeter appears 



454 MINEEAL AND LAKE PIGMENTS. 

somewhat complicated, yet it yields the most accurate results 
with the smallest expenditure of labour and time. The 
value of a colouring material may also be estimated by pre- 
paring the pure colouring matter from a weighed quantity. 
This process is lengthy, demands considerable practice, and 
only gives good results when it is carried out with the 
most painful accuracy. The colouring matters in question 
are precipitated by lead salts. If the dye-wood extract con- 
tained colouring matter alone its amount could be found by 
observing the volume of a lead solution of known strength 
required to precipitate the colouring matter completely. 
The decoctions, however, contain other substances which 
form lead compounds, and are precipitated together with 
the colouring matter, so that if the precipitate were regarded 
as the pure lead compound of the colouring matter a very 
inaccurate result would be obtained. In order to obtain 
results with some pretensions to accuracy the lead compound 
of the colouring matter must be purified. The impure pre- 
cipitate is washed and suspended in water, through which 
sulphuretted hydrogen is passed until all the lead is pre- 
cipitated as lead sulphide, which is filtered off, excess of 
sulphuretted hydrogen expelled by boiling, and the solution 
again precipitated by a lead salt. This precipitate may be 
regarded without considerable error as the lead compound of 
the colouring matter. The weight of colouring matter con- 
tained in the quantity of wood used can be calculated from 
the weight of the dry precipitate. This method is somewhat 
complicated and tedious ; the results are interior m accuracy 
to those obtained by means of the colorimeter, which instru- 
ment furnishes the most suitable method for testing dye-woods 
for practical purposes. 

A thorough knowledge of chemistry is indispensable to 
the colour manufacturer who wishes to carry on his business 
on any extensive scale. It enables him to match any sample 



THE TESTING OF DYE-WOODS. 455 

of colour submitted to him and to test his raw materials 
with ease. We have indeed given in a section of this book 
simple methods by which the majority of commercial pig- 
ments can be tested with tolerable accuracy by means of a 
few reagents, and for ordinary purposes these methods are 
sufficient. But when an accurate examination of a pigment 
is required, it must be conducted by the ordinary processes of 
analytical chemistry. The colour manufacturer has not only 
to carry out these occasional examinations, but has frequently 
to test certain raw materials which he uses in large quantities. 
Soda may be taken as an example. An estimation of the 
percentage of sodium carbonate in this substance is an 
exceedingly simple matter to the chemist, but can hardly be 
carried out without a knowledge of chemistry. 

A manufacturer without chemical knowledge, who is 
carrying on an industry which, like the manufacture of 
colours, rests entirely upon a chemical foundation, will 
constantly be compelled to seek advice from a scientific 
chemist. Many pigments can be made according to a settled 
formula, but the results of working strictly according to the 
formula, without a knowledge of the reasons for the opera- 
tions, can only be satisfactory whilst no irregularity occurs. 
The least irregularity places those who work blindly in a 
completely helpless position, for they do not know what is 
wrong and cannot remove the hindrance. 

In the manufacture of pigments certain by-products are 
produced. These can be utilised by a manufacturer possessed 
of chemical knowledge, whilst they are simply thrown away 
by many, thus making the manufacture of the particular 
pigment far more expensive than when the by-product is 
also made valuable. Strictly speaking, there are no worth- 
less by-products in making pigments every liquid obtained 
in precipitating a colour might be further utilised. Salt 
solutions can only be regarded as worthless by-products 



456 MINEKAL AND LAKE PIGMENTS. 

when the cost of separating the salt from the solution 
would be greater than the value of the product. Thus we 
cannot conclude this section of the work without again 
insisting that the study of chemistry is indispensable to the 
colour maker, since his industry is chemical from beginning 
to end. The colour maker who works simply by recipes 
will never raise himself above the position of an ordinary 
workman, who does what he is told without thinking of 
what he is doing. The smallest mistake in carrying out the 
process generally results in the complete failure of the whole 
operation and thus causes the manufacturer material loss. 



CHAPTEK LXXIII. 

JTHE DESIGN OF A COLOUR WORKS. 

IN the establishment of a colour works several conditions 
are necessary. The most important is the supply of water 
in sufficient quantity and purity. It has already been stated 
that many pigments cannot be made with water containing 
much organic matter or salts, since the dissolved substances 
affect the shade. This action not only takes place in the 
formation of the colour, but is also unpleasantly manifest 
when it is washed. The delicate lakes are discoloured by 
organic matter in the water, and are so changed by any 
considerable quantity of lime that the alteration in shade is 
clearly perceptible after continued treatment. If the water 
contains but arvery small quantity of iron, the preparation of 
some pigments is made quite impossible, since and this 
is especially the case with the lakes the ferric oxide is 
precipitated together with the pigment, and in consequence 
of its characteristic colour imparts to it an ugly shade. 
Thus in choosing a site for a colour works the available 
water supply must be carefully examined. If it is not 
sufficiently pure or in sufficient quantity, the position must 
be regarded as unsuitable. 

No colour maker, although working on the largest scale, 
is in a position to make all the materials he requires. With 
the continual development of the chemical industries, the 
number of these substances which can be made economically 



458 MINERAL AND LAKE PIGMENTS. 

in the colour works continually diminishes. It is far more 
advantageous to obtain them from works in which they are 
made on the large scale. Many of these substances are 
required in large quantity, so that a site should be chosen in 
direct communication with the railway, so that the cost of 
carriage is diminished, and also the cost of distributing the 
manufactured materials. This is especially necessary for 
materials which have a low value and consequently can bear 
no high cost of carriage. 

In regard to the space required by a colour works, no 
actual dimensions can be given, since these vary with the 
extent of the business, and with the pigments produced. 
In the price lists of large colour works all the commercial 
pigments are generally quoted, but they are rarely if ever 
actually made in one works, but are obtained at a lower price 
from other establishments, which make a speciality of certain 
pigments, and by working on a large scale can produce them 
at such a cost that the smaller manufacturer is not in a 
position to compete with them. Thus there are works in 
which only white lead or ultramarine is made. 

If a colour manufacturer is in the fortunate position of 
placing his works on a river, he has not only an unrestricted 
water supply, but may also be able to use water power, the 
cheapest of all powers. When water power is available, it 
will be used to raise the large volumes of water required and 
to move the machinery for grinding the raw materials, 
rasping dye-woods, etc. In a colour works of any size a 
boiler is required ; if water power is not used, it must be of 
sufficient power to give steam for driving the engine, for 
boiling liquids and for heating the drying stoves. The 
boiler is of great advantage in providing steam for dissolving 
salts, extracting dye-woods and boiling liquids. When liquids 
are boiled by steam there is considerable economy in that the 
majority of the boiling tubs can be of wood, which is pro- 



THE DESIGN OF A COLOUR WORKS. 459 

vided with a protective coating for liquids which attack 
wood. Thus there is economy in dispensing with large 
metal pans and with the fire-places in which they would be 
built, and also in the course of time there is considerable 
saving in fuel. 

For a well-equipped colour works a drying stove, in which 
the pigments can be thoroughly dried, is important. It is 
most convenient to heat drying stoves with steam. The 
temperature can be easily regulated by increasing or diminish- 
ing the supply of steam. For drying pigments which would 
not be injured by considerable increase of temperature, the 
drying stove may be heated by a fire. 

In a colour works in which many pigments are made 
sulphuretted hydrogen is frequently required. Since all lead 
pigments are blackened by this gas, the greatest care is 
required in using it. Also the poisonous nature of sulphur- 
etted hydrogen is generally under-estimated. In working 
with sulphuretted hydrogen, the apparatus depicted in Fig. 3 
should be used. It should be placed in a position, such as a 
closed yard, in which escaping gas will be harmless. If this 
cannot be done the precipitation should be accomplished in 
closed vessels, from which a pipe should carry the gas to a 
fire, where it will be burnt to sulphur dioxide and water. 

In most cases the manufacturer of colours makes but 
a certain number of pigments and rarely or never all which 
are mentioned in his price lists. The dimensions of the 
establishment will be in accordance. In commencing a new 
colour works it is advisable from purely financial reasons 
to produce at first a limited number of colours, but these 
in perfection. By many experiments and diligent study 
of chemistry a colour manufacturer may hope to compete 
successfully in so difficult a branch of chemical technology 
as the manufacture of colours. 



CHAPTEE LXXIV. 

COMMERCIAL NAMES OF PIGMENTS. 

IN commerce the pigments are found under the most differ- 
ent names, the most common of which have been given 
together with the description of the pigment. No regularity 
can be found in the names chosen for the different pigments ; 
quite arbitrary designations have been taken. Pigments are 
most commonly named after places for example, Prussian 
blue, Paris blue, Bremen green ; also after the discoverer 
Turnbull's blue, Hatchett brown. Whilst these names 
give the place of production or the name of the discoverer, 
and thus have some foundation, there are many others 
for which no reason can be assigned, e.g., King's yellow. Cer- 
tain names are based upon the chemical composition of the 
pigment. These should be used by preference, but now that 
the expressions white lead, chrome yellow and Chinese blue 
have become common no one would think of speaking of 
basic lead carbonate, lead chromate or ferric ferrocyanide. 
The confusion in the nomenclature of colours is increased 
by placing pigments which possess English names upon the 
market under French, German or Latin names, which are 
often sadly mutilated. This is more the case in Germany 
than in England. 

It may easily happen that a reader of a work on colour 
making might search in vain for a pigment whose name he 
had somewhere heard, whilst the book contained a description 



COMMERCIAL NAMES OF PIGMENTS. 461 

of the colour and its properties, but under another name. 
To remove this difficulty it has been thought necessary to 
collect the names of the different pigments, which are con- 
tained in the following table. The French and German 
names are also given. The most usual names are printed in 
italics. 

WHITE PIGMENTS. 

Basic Lead Carbonate, White lead, flake white. 

Ceruse, blanc de plomb, blanc d'argent, blanc de neige, 
fleur de neige, blanc de Venise. 

Bleiweiss, Schieferweiss, Schneeweiss, Silberweiss, Krem- 
serweiss, Kremnitzerweiss, Berlinerweiss, Venetianer- 
weiss, Hollanderweiss, Hamburgerweiss. 

Lead Oxychloride. Pattisons white lead. 

Blanc de Pattison, blanc d'Angleterre. 
Bleiweiss, Pattisonweiss, englisches Patentweiss. 

Lead Sulphate. Lead bottoms. 
Ceruse de Mulhouse. 
Bleiweiss, Vitriolweiss. 

Barium Sulphate, -Enamel white, permanent white, blanc 
fixe, baryta white. 

Blanc fixe, blanc permanent. 

Permanentweiss, Barytweiss, Schneeweiss, Mineralweiss, 
Neuweiss. 

Zinc Oxide. Zinc white, permanent white, flowers of zinc. 
Blanc de zinc, oxyde de zinc, fleur de zinc. 
Zinkweiss, Zinkblumen, weisses Nichts, Ewigweiss. 

Basic Bismuth Nitrate. Pearl white, bismuth white, Spanish 
white. 

Blanc d'Espagne, blanc de bismuth, blanc de fard. 
Wismuthiveiss, Spanischweiss , Perlweiss, Schminkweiss. 



462 MINERAL AND LAKE PIGMENTS. 

YELLOW PIGMENTS. 

Lead Chromate, Chrome, Chrome yellow. 
T j 7 < yMv r ^> Qfl-Z-Ai - 

Jaune de chrome, jauneu on. 

Chromgelb, Konigsgelb, Citronengelb, Neugelb, Pariser- 
gelb, Leipzigergelb, Kolnergelb, Zwickauergelb, ame- 
rikanisches Gelb. 

Lead Oxide. Litharge, massicot. 

Bleigldtte, Glatte, Massicot. 

Lead Oxychloride. Patent yellow. 

Jaune mineral, jaune brevete", jaune de Montpellier. 
Casselergelb, Veronesergelb, Mineralgelb, Patentgelb, Eng- 
lischgelb, Parisergelb, Montpelliergelb. 

Lead Antimoniate. Naples yellow. 

Jaune de Naples, jaune d'antimoine. 

Neapelgelb, Antimongelb. 
Giallolino, Giallo di Napoli. 

Barium Chromate. -Lemon yellow, baryta yellow, yellow 
ultramarine, permanent yellow. 

Barytgelb. gelbes Ultramarin, Chromgelb. 

Zinc Chromate. Zinc yellow, zinc chrome. 

Jaune de zinc, jaune permanent, jaune bouton d'or, 
jaune de chrome inalterable, jaune d'outremer. 
Zinkgelb, Chromgelb. 

Cadmium Sulphide. Cadmium yellow, aurora yellow. 
Jaune brillant. 

Cadmiumgelb. 

Basic Mercuric Sulphate, Turpeth mineral, mercury yellow. 
Jaune de mercure. 
Mercurgelb, Konigsgelb, mineralischer Turpeth. 

Cobalt Potassium Nitrite. Aureolin, cobalt yellow. 
Jaune indien. 
Kobaltgelb. 



COMMEKCIAL NAMES OF PIGMENTS. 463 

Arsenic Disulphide and Trisulphide. Realgar, orpiment, 
King's yellow. 

Orpiment, jaune royal. 

Realgar, Auripigment, Rauschgelb, Kauschroth, Konigs- 
gelb, Chinagelb, Persischgelb, Spanischgelb. 

Stannic Sulphide, Mosaic gold. 

Musivgold. 

YELLOW LAKES. 

Dutch pink, French yellow, stil de grain, jaune 
d' Avignon, jaune frangais, Schuttgelb. 
Yellow lake, jaune de gauche, Waulack. 
Indian yellow, jaune indien, Purre, Indischgelb. 
Gamboge, gomme-goutti, Gummigutt, Gummigutti. 

KED PIGMENTS. 

Basic Lead Chromate. Chrome red, orange chrome, Persian 
red, Derby red, Chinese red. 

Jaune d'or, jaune orange, pate orange. 
Chromro t h , Chromorange . 

Mercuric Sulphide* Vermilion, cinnabar. 
Vermilion, cinabre. 
Zinnober, Vermilion, Chinesischroth, Patentroth. 

Mercuric Iodide. Scarlet. 
Ecarlate. 
Jodinroth, Scharlachroth. 

Ferric Oxide. Rouge, colcothar, Indian red, Venetian red, 
crocus, caput mortuum, Mars red. 

Rouge des Indes, rouge de Mars, rouge d'Angle- 
terre. 

Englischroth, Erigelroth, Berlinerroth, Konigsroth, 
Kaiserroth, Neapelroth, Indischroth, Persischroth, 
Eisensafran, Todtenkopfroth, Marsroth. 



464 MINEEAL AND LAKE PIGMENTS. 

Cobalt Phosphate or Arsenate. 

Chaux metallique. 
Kobaltrosa. 

KED LAKES. 

Cochineal Lakes, Carmine, crimson lake. 
Laque carminee. 

Carmin, Cochenilleroth, Miinchenerlack, Wienerlack, 
Florentinerlack, Pariserlack. 
Lac Dye. 

Lack-lack, Lack-dye. 

Madder Lakes. Rose madder, purple madder, madder car- 
mine. 

Laque de garance. 

Krapplack, Wienerlack, Krappcarmin, Garancincarmin. 
Red Wood Lakes Eose pink, Florentine lake. 

Laque en boules, laque de Vienne, laque de Venise, 
laque de Florence. 

Kugellack, Miinchenerlack, Wienerlack, Berlinerlack, 
Florentinerlack, Venezianerlack, Neulack. 
Carthamine. Safflower, Spanish red. 

Rouge de carthame, rouge de Chine, rouge d'or, rouge 
en ecailles, rouge vegetal, rouge de Portugal. 

Safflorcarmine, Safflorroth, Tassenroth, Tellerroth, Vege- 
talroth. 

BLUE PIGMENTS. 

Ferric Ferrocyanide, Prussian blue, Chinese blue, Paris blue, 
Berlin blue, Brunswick blue. 

Bleu de Prusse, bleu de Paris, bleu de Berlin, bleu 
d'Anvers. 

Pariserblau, Berlinerblau, Preussischblau, Sachsisch- 
blau, Neublau, Oelblau, Wasserblau, Mineralblau, 
Erlangerblau, Zwickauerblau, Waschblau, Louisen- 
blau, Eaymondblau. 



COMMEECIAL NAMES OF PIGMENTS. 465 

Ferrous Ferricyanide, Turnbull's blue. 

Bleu de Turnbull. 

Turnbull's Blau. 

(The names given under ferric ferrocyanide are also 
used.) 
Ultramarine, Lime blue, Koyal blue. 

Outremer, bleu d'azur. 

Ultramarin, Azurblau, Lazurblau, Lapis lazuli-Blau. 
Copper Hydroxide or Carbonate (singly or together). Blue 
verditer, lime blue, mountain blue. 

Bleu de montagne, bleu de chaux, bleu de cuivre, cendres 
bleues, bleu de Payen. 

Bergblau, Mineralblau, Oelblau, Kalkblau, Neubergblau, 
Kupferblau, Bremerblau, Steinblau, Hamburgerblau, 
Neuwiederblau, Casselerblau. 

Cobalt Oxide Alumina Compound. Cobalt blue, Thenard's 
blue. 

Bleu de Thenard, bleu de cobalt. 

Kobaltblau, Thenard'sches Blau, Kobalt-ultramarin, 
Konigsblau, Leydenerblau, Leithnerblau, Wienerblau, 
Cteruleum. 
Cobalt Potassium Silicate, Smalts. 

Bleu de smalt, bleu d'azur, bleu de Saxe. 

Smalte, Schmalte, Blaufarbenglas, Sachsischblau, 
Streublau, Konigsblau, Kaiserblau, Azurblau, Eschel. 

BLUE ORGANIC PIGMENTS. 

Indigo Sulphonates, Indigo carmine, soluble indigo. 
Bleu de Saxe. 

Indigocarmin, Carminblau, pracipitirter Indigo, Car- 
minblau. 
Indigo Lake, Bleu d'Angleterre, bleu de Hollande. 

Neublau, Waschblau, Hollanderblau, Englischblau, 

Tafelindigo. 

30 



466 MINEEAL AND LAKE PIGMENTS. 

GEEEN PIGMENTS. 

Copper Carbonate, Mountain green, Hungarian green, lime 
green. 

Vert de montagne, vert mineral, vert de cuivre, vert 
de Hongrie. 

Berggrun, Kupfergriin, ungarisches Griin, Tirolergriin, 
Malachitgriin, Mineralgriin, Schiefergriin, Glanzgrim, 
Staubgrlin, Wiesengriin, Apollogriin, Wassergriin, Oel- 
griin, Alexandergrun. 

Copper Arsenite, Scheeles green, verditer, lime green, mineral 
green. 

Vert de Suede, vert de Scheele. 

Scheel'sches Grim, Schwedisches Griin, Mineralgriin, 
Braunschweigergriin, Neuwiedergriin, Erdgriin, Aschen- 
griin. 

Copper Aceto-Arsenite, Emerald green. 

Vert de Vienne, vert de Mitis, vert brevete. 
Schweinfurter Grun, Mitisyrun, Wiesengrun, Entjliscli- 
grun, Patentgrun, Hermann's Griin, Papageigriin, Kaiser- 
griin, Konigsgri'm, Wienergriin, Kirchbergergriin, Leip- 
zigergriin, Zwickauergriin, Baslergriin, Parisergriin, 
Neuwiedergrun, Wiirzburgergnin, Originalgri'm, Jas- 
niigergrim. 

Copper Stannate, 

Gentele's Grun, Zinngriin. 
Copper Oxychloride, 

Kuhlmann's Griin, Eisner's Grun, giftl'reies Griin. 

Copper Borate. 

Borgrlin, Kupfergriin, giftfreies Kupfergriin. 

Copper Acetate, Verdigris, distilled or crystallised verdigris. 
Vert de tjris, vert de gris natnrel, vert de gris dis- 
tille, vert de gris en grappes. 



COMMEECIAL NAMES OF PIGMENTS. 467 

Grunspan, destillirter, franzosischer, deutscher, pra- 
cipitirter or krystallisirter Grunspan. 

Chromium Oxide and Hydroxide, Guignet's green, chrome 
green, lime green, viridian. 

Vert de chrome, vert Pannetier, vert Guignet, vert 
de soie, vert emeraude, vert naturel, vert virginal. 

Chromgrun, griiner Zinnober, Laubgriin, Smaragd- 
griin, Deckgriin, Myrthengriin, Permanentgriin, Ameri- 
kanergriin, Neapelgriin, Gothaergriin, Guignet's Griin, 
Mittler's Griin, Pannetier's Griin, Chromgrun in Lack, 
Tiirkisgriin, Seidengriin, Naturgriin. 

[The green produced by mixing chrome yellow with 
Prussian blue is known in England as chrome green, 
Brunswick green, oil green, etc., and on the continent 
by the names given above for chromium oxide.] 
Chromium Phosphate, Arnaudan's green. 

Vert Arnaudan, vert de Plessy. 

Arnaudan's, Plessy's, Schnitzer's Griin. 
Cobalt Oxide-Zinc Oxide, Cobalt green, Kinmann's green. 

Vert de Rinmann, vert de cobalt. 

Kobaltgriln, Rinmann's Griin, Zinkgriin, permanenter 
griiner Zinnober. 
Barium Manganate, Rosenstiehl's green. 

Vert tiges de roses. 

Mangangriin, EosenstiehV s Griin, Bottger's Griin. 

VIOLET PIGMENTS. 
Chromic Chloride. 

Chrombronze, Permaiientbronze, Tapetenbronze. 
Manganese Phosphate, Nuremberg violet. 

Manganviolett, Niirnberger Violett. 

BROWN PIGMENTS. 
Lead Peroxide. 

Bleibraun, Flohbraun. 



468 MINERAL AND LAKE PIGMENTS. 

Manganic Oxide and Peroxide. Manganese brown. 
Brun de Mangane, bistre mineral. 
Manganbraun, Bisterbraun, Mineralbister, Kastanien- 
braun, Braunsteinbraun. 

Copper Potassium Ferrocyanide, Hatchett brown. 
Brun de Prusse. 

Hatchett's Braun, Kupferbraun, Chemischbraun, 
Breslauerbraun. 

Ferric Oxide. Brown ochre, Mars brown. 

Eisenbraun, Van Dyck Braun, Ockerbraun, Siena- 
braun. 

Prussian brown is Prussian blue decomposed by 
heat (see page 280). 

Chrome brown and cobalt brown (see pages 280, 281). 

BLACK PIGMENTS. 

Carbon, -Ivory black, bone black, Frankfort black, vine 
black, ve/etable black, drop black, carbon black, lamp 
black. 

Noir d'ivoire, noir de Frankfort, noir de Cologne, 
noir d'Allemagne. 

Kienruss, Flatterruss, Kussschwarz, Kebenschwarz, 
Hefeschwarz, Bemschwarz, Spodium, Elfenbein- 
schwarz, Frankfurterschwarz, Pariserschwarz, Wiener- 
schwarz, Lampeiischwarz, Oelschwarz, Spanisch- 
schwarz, Druseiischwarz. 

Carbon mixed with Other Substances, 

Neutral tint, composition black, neutral black. 
Teint neutre, noir de composition. 
Compositionsschwarz, Neutraltinte, Naturaltinte. 

Indian ink. 
Encre de Chine. 
Tusche. 



APPENDIX. 

THE CONVERSION OF METRIC INTO ENGLISH WEIGHTS 
AND MEASURES. 

The conversion of metric into English weights and 
measures can be readily accomplished by means of the 
following relationships : 

MEASURES OF LENGTH. 

1 metre = 100 centimetres = 39-37 inches. 
1 foot =0-3381 metre. 

MEASURES OF CAPACITY. 

1 litre = 1,000 cubic centimetres = -2209 gall. 
1 gall. = 4-5436 litres. 

MEASURES OF WEIGHT. 

1 kilogramme = 1,000 grammes = 2-205 Ib. 

1 gramme = 15-443 grains. 

1 cwt. = 50-802 kilogrammes. 

CENTIGRADE AND FAHRENHEIT THERMOMETER SCALES. 

To convert temperatures expressed in Centigrade degrees 
to Fahrenheit degrees multiply by 9, divide by 5, and add 
32. 

To convert temperatures expressed in Fahrenheit de- 
grees to Centigrade degrees subtract 32, multiply by 5, 
and divide by 9. 



INDEX. 



ACID, acetic, 30. 

- carbonic, 30. 

- hydrochloric, 25. 

- nitric, 28. 

- oxalic, 31. 

- sulphuric, 27. 

- tartaric, 31. 
Acids, 25. 
Alizarin, 372. 
Alkalis, 32. 
Alkanet, 368. 
Alum, ammonia, 48. 

- Roman, 46. 

- soda, 47. 
Alumina, 49. 

- gold purple, 193. 
Aluminium compounds, 42. 

- sulphate, 43. 
Alums, 44. 
Ammonia, 23. 

- cochineal, 362. 
Ammonium chloride, 25. 

- sulphide, 25. 
Annaline, 131. 
Antimony blue, 179. 

- compounds, 59. 

- oxychloride, 129. 

- trioxide, 129. 

- vermilion, 178. 

- yellow, 151. 
Antwerp blue, 203. 
Apparatus, washing, 120. 
Appendix, 469. 

Aqua regia, 29. 
Archil, 381. 

Arsenic compounds, 59. 
Asphaltum, 414. 
Aureolin, 156. 

BARIUM carbonate, 42. 
chloride, 41. 



Barium compounds, 41. 

- green, Bottger's, 271. 

- sulphate, 116. 

- yellow, 152. 
Barytes, 116. 
Bismuth compounds, 59. 

- white, 130. 
Bistre, 284. 
Black, bone, 290. 

- charcoal, 286. 

- chrome, 318. 

- chrome-copper, 318. 

- enamel, 325. 

- ivory, 290. 

- lamp, 307. 

- neutral tint, 318, 

- pigments, 285, 444. 

pine, 312. 

- soot, 294, 313. 

- vine, 288. 
Blowpipe, 436. 
Blue, antimony, 179. 

- Antwerp, 203. 

- Bremen, 226. 

- Brunswick, 200. 

- Chinese, 196, 201. 

- chrome, 267. 

- cobalt, 230. 

- copper pigments, 226. 

- Egyptian, 250. 

enamels, 323. 

- lakes, 390, 397. 

- lime, 228. 

- manganese, 272. 

- mineral, 200. 

- mineral pigments, 194. 

- molybdenum, 239. 

- Neuberg, 227. 

- oil, 228. 

- Payen's, 228. 

pigments, examination of, 441. 



472 



INDEX. 



Blue, Prussian, 199. 

Soluble Prussian, 200. 

- Tessie du Motay's, 293. 

- tungsten, 238. 

- Turnbull's, 203. 

- ultramarine, 204. 

- pale, 223. 

verdigris, 252. 
Bone black, 290. 
Bottger's barium green, 271. 
Brasilein, 384. 

Brasilin, 384. 
Brazil wood, 384. 
Bremen blue, 226. 

- green, 226. 
Brocade pigments, 341. 
Bronze, electrolytic copper, 337. 

- pigments, 329. 

- pigments, tungsten, 338. 
Brown, chrome, 280. 

- cobalt, 281. 

- copper, 280. 

- decomposition products, 283. 

- Hachett's, 280. 

iron, 280. 

- lead, 279. 

- manganese, 279. 

- mineral pigments, 279. 

- organic pigments, 414. 

- pigments, examination of, 443. 

- Prussian, 280. 

- pyrolusite, 279. 
Brunswick green, 243. 

CADMIUM compounds, 53. 

- yellow, 153. 
Cseruleum, 231. 
Calcium carbonate, 40. 

- compounds, 39. 

- hydroxide, 39. 

- oxide, 39. 

- phosphate, 40. 

- sulphate, 40. 
Carajuru, 379. 
Carbon, 30. 
Carmine, 354, 357. 

- indigo, 394. 

- madder, 376. 

- safflower, 367. 
Carthamine red, 366. 
Casselmann's green, 249. 
Cassel yellow, 148. 
Cassius, purple of, 190. 



Caustic potash, 33. 
Caustic soda, 37. 
Charcoal blacks, 286. 
Charvin's green, 412. 
Chica red, 379. 
Chinese blue, 196, 201. 

- green, 411. 

- vermilion, 170. 
Chlorine, 20. 
Chlorophyll, 409. 
Chromaventurine, 266. 
Chrome alum, 35. 

black, 318. 

- blue, 267. 

- brown, 280. 

- copper black, 318. 

- green, 260, 274. 

- Eisner's, 274. 

- lake, 264. 

- red, 186. 

- yellow, 134. 

- cadmium, 153. 

- calcium, 151. 

- pale, 139. 

- zinc, 152. 
Chromic chloride, 276. 
Chromium compounds, 58. 

- oxide, 260. 

- stannate, 189. 
Chrysoan, 162. 
Cobalt arsenate, 189. 

- blue, 230. 

- brown, 280. 

- compounds, 56. 

- green, 268. 

- magnesia red, 188. 

- red, 188. 

- ultramarine, 230. 

- zinc phosphate, 232. 
Cochineal, 354. 
Colorimeter, 450. 

Colours, simple and mixed, 71. 
Colour works, design of, 457. 
Commercial names of pigments, 

460. 

Confectionery colours, 427. 
Copper acetate, 65. 

- arsenite, 241. 

- borate, 250. 

- brown, 280. 

- carbonate, 240. 

- compounds, 65. 

- hydroxide, 229. 



INDEX. 



473 



Copper nitrate, 65. 

- oxychloride, 243. 

- silicate, 250. 

- stannate, 248. 

sulphate, 65. 

- violet, 278. 
Covering power, 10. 
Crayons, 423. 
Cream of tartar, 34. 
Cudbear, 382. 
Curucuru, 379. 

DESIGN of a colour works, 457. 
Dextrine, 420. 
Drying oils, 429. 

- stove, 459. 
Dutch pink, 348. 
Dye-woods, 449. 

EGYPTIAN blue, 250. 
Eisner's chrome green, 274. 

- green, 249. 
Emerald green, 244, 264. 
Enamel colours, 319. 
Enamel white, 116. 
Enamels, black, 325. 

- blue, 323. 

green, 324. 

- red, 322. 

- violet, 324. 

- white, 320. 

- yellow, 322. 

Examination of pigments, 434. 
Extracts, 398, 402. 

FERNAMBUCO wood, 384, 386. 
Ferric oxide pigments, 180. 
Ferrous chloride, 54. 

- sulphate, 54. 
Filter press, 124, 125. 
Florentine lake, 361. 
French purple, 38. 
Fustic lake, 351. 

GAMBOGE lake, 349. 

- prepared, 350. 
Garanceux, 372. 
Garancin, 371. 

Gardinia grandiflora, colouring 

matter of, 353. 
Glucose, 422. 
Gold, compounds of, 69. 



Green, Arnaudan's, 265. 

- Bremen, 226. 

- Brunswick, 243. 
Green, Casselmann's, 249. 

- Charvin's, 412. 

- Chinese, 411. 

- chrome, 260, 274. 

- cobalt, 268. 

- Eisner's, 249. 

- emerald, 244, 264. 

- enamel, 324. 

- Guignet's, 264. 

- Kuhlmann's, 249. 

lakes, 409. 

- leaf, 265. 

- lime, 250. 

manganese, 270. 

- mineral pigments, 240. 

- natural, 275. 

- Neuwied, 243. 

- non-arsenical, 275. 

- patent, 250. 

- pigments, compounded, 273. 

- pigments, examination of. 442. 

- Plessy's, 266. 

- Rosenstiehl's, 270. 

- sap, 410. 

- Scheele's, 241. 

Schnitzer's, 266. 

- silk, 275. 

- Turkish, 265. 

- verditer, 243. 

- Vienna, 248. 
Guignet's green, 264. 
Gum Arabic, 420. 
Guyard's violet, 278. 
Gypsum, 40. 

HACHETT'S brown, 280. 
Hsematei'n, 406. 
Hsematoxylin, 406. 
Humins, 283. 
Hydrometer, 24. 

INDIAN ink, 316. 

- madder, 378. 

- red, 185. 

- yellow, 352. 
Indigo, 390. 

- carmine, 394. 
Introduction, 1. 
Iron brown, 280. 

- compounds, 54. 



30* 



474 



INDEX. 



Iron red, 182. 
Ivory black, 290. 

KUHLMANN'S green, 294. 

LAC, 363. 

dye, 363. 

Lake, blue, 390, 397. 

- Florentine, 361. 

- fustic, 351. 

- gamboge, 349. 

- green, 409. 

- quercitron, 351. 

- weld, 349. 
Lakes, 6, 9, 343. 

examination of, 445. 

- madder, 375. 

- red, 354. 

- yellow, 348. 
Lamp black, 307. 
Lead acetate, 62. 

- antimonate, 115. 

antimonite, 115. 

- arsenite, 159. 

- brown, 279. 

- chloride, 64. 

chromate, 134. 

- compounds, 60. 

- iodide, 154. 

- monoxide, 143. 

- nitrate, 61. 

- orange, 146. 

- oxychloride, 113. 

- red, 144. 

- sulphate, 61, 112. 

- sulphite, 114. 

tungstate, 128. 

- white, 73, 74, 75, 77. 

- white, hard and soft, 191. 
Leaf green, 265. 

Lichens, 380. 
Lime, 39. 

- blue, 228. 

- green, 250. 
Litharge, 143. 
Lithophone, 119. 
Litmus, 382. 
Logwood, 398. 
Lokao, 44. 

MADDER, 370. 

- carmine, 376. 

extract, 372. 



Madder, Indian, 378. 

- lakes, 375. 
Magnesia, gold purple, 192. 

- white, 130. 

Magnesium carbonate, 41. 
Manganese blue, 272. 

- brown, 279. 

compounds, 59. 
green, 270. 

- sulphate, 56. 

- violet, 278. 

- white, 130. 
Manganous oxide, 272. 
Mangit, 378. 

Mars yellow, 155. 

Massicot, 143. 

Mercuric ammonium chloride, 175. 

- chloride, 68. 

- iodide, 176. 

nitrate, 68. 

- sulphite, 163. 
Mercurous chloride, 68. 

- nitrate, 67. 
Mercury compounds, 67. 

- yellow, 158. 
Metallic pigments, 326. 
Metals, heavy, 51. 
Mills, paint, 430. 

- white lead, 86. 
Mineral lake, 278. 
Molybdenum blue, 239. 

- compounds, 59. 
Montpellier yellow, 148. 
Mosaic gold, 160. 

NAPLES yellow, 149. 
Natural green, 275. 
Neuberg blue, 227. 
Neutral tint black, 318. 
Neuwied green, 243. 
Nickel compounds, 56. 

- yellow, 157. 
Non-arsenical green, 275. 

OIL blue, 228. 
Orange lead, 146. 

- Mars, 155. 
Orcei'n, 300. 
Orcinol, 380. 
Orpiment, 159. 

PAINT mills, 430. 
Patent green, 250. 



INDEX. 



475 



Payen's mountain blue, 228. 
Permanent yellow, 152. 
Pigments, artificial mineral, ( 

- commercial names of, 460. 

earth, 6. 
Pine black, 312. 
Pink, Dutch, 348. 
Plessy's green, 266. 
Poisonous pigments, 13. 
Potassium alum, 45. 

- bichromate, 35. 
-r- bitartrate, 34. 

- carbonate, 32. 

- compounds, 32. 

- ferricyanide, 35. 

- ferrocyanide, 35. 

hydroxide, 33. 

- nitrate, 34. 

- sodium chromate, 35. 
Precipitation, 7. 
Preparation of pigments, 429. 
Prepared gamboge, 350. 
Prussian blue, 199. 

- brown, 2.-0. 

Purple, alumina gold, 193. 

- French, 381. 

- magnesia gold, 192. 

- of Cassius, 190. 

red, 189. 
Purpurin, 373. 
Purree, 352. 
Pyrolusite brown, 280. 

QUERCITRON lake, 351. 
Quicklime, 39. 

REALGAR, 159. 

Red, carthamine, 366. 

- chica, 379. 

- chrome, 186. 

- cobalt, 188. 

- magnesia, 188. 

- enamels, 322. 

- haematite, 180. 

- Indian, 185. 

- iron, 182. 

lakes, 354. 

lead, 144. 

- Mars, 155. 

- mineral pigments, 163. 

- pigments, examination 

440. 

purple, 189. 



of, 



Red wood lakes, 384. 
RosenstiehPs green, 270. 

SAFFLOWER, 366. 

- carmine, 367. 
Saffron, 352. 

Sal ammoniac, 25. 
Salt, common, 38. 
Sandalwood, 388. 
Sap colours, 416. 

- green, 410. 
Scheele's green, 241. 
Schnitzer's green, 266. 
Sepia, 414. 

Shell gold, 326. 
Shell silver, 327. 
Siderin yellow, 156. 
Silk green, 275. 
Silver chromate, 189. 

- compounds, 69. 

- imitation, 328. 
Smalts, 233. 
Sodium chloride, 38. 

- hydroxide, 37. 

- salts, 37. 

- thiosulphate, 38. 
Soluble Prussian blue, 200. 
Soot black, 313. 

- pigments, 294. 
Stannic chloride, 59. 
Stannous chloride, 59. 
Stove, drying, 459. 
Sulphuretted hydrogen, 26. 
Swedish green, 241. 



du Motay's blue, 239. 
Thallium pigments, 159. 
Thenard's blue, 230. , 
Tin compounds, 59. 

- violet, 278. 

- white, 130. 
Tragacanth, 420. 
Tungsten blue, 238. 

- bronze pigments, 338. 

- compounds, 59. 

- yellow, 157. 
Turkish green, 265. 
Turnbull's blue, 203. 
Turner's yellow, 149. 
Turpeth mineral, 158. 

ULTRAMARINE, 3, 204, 211. 
artificial, 206. 



476 



INDEX. 



Ultramarine, cobalt, 230. 

natural, 205. 

- violet, 219. 

- yellow, 152. 

VANADIUM compounds, 59. 
Vegetable bronze pigments, 339. 
Verdigris, 252. 

- blue, 252. 

- distilled, 255. 

- German, 258. 
Verditer, green, 243. 
Vermilion, 3, 163, 166. 

- antimony, 178. 

- Chinese, 170. 

- chrome, 186. 
Vienna green, 248. 
Vine black, 288. 
Violet copper, 278. 

- enamel, 324. 

- Gu yard's, 278. 

- manganese, 278. 

- mineral pigments, 276. 

Nuremberg, 278. 

- tin, 278. 

- ultramarine, 219. 
Vitriol, blue, 65. 

green, 54. 

oil of, 27. 

WASHING apparatus, 120. 
Water, 16. 

- colours, 419. 

- moist, 422. 

examination of, 20. 

- hard, 17. 

- iron in, 18. 

organic matter in, 18. 
Weld lake, 349. 
White, bismuth, 130. 

enamel, 116, 121, 320. 

lead, 73, 74, 75. 



White lead mills, 861. 

- magnesia, 131. 

- manganese, 130. 

- mineral pigments, 72. 

- patent, 73. 

- permanent, 116, 121. 

- pigments, examination of, 437. 

- tin, 130. 

- tungsten, 128. 

- zinc, 126. 
Witherite, 42, 117. 

YELLOW, antimony, 151. 

- barium, 152. 

- cadmium chrome, 153. 

- calcium chrome, 151. 

- Cassel, 148. 

- chrome, 134. 

- enamels, 322. 

- English, 149. 

- Indian, 352. 
lakes, 348. 

- Mars, 155. 

- Mercury, 158. 

- mineral pigments, 133. 

- Montpellier, 148. 

- Naples, 149. 

- nickel, 157. 

- pale chrome, 139. 

- pigments, examination of, 439. 

- Siderin, 156. 

- tungsten, 157. 

- Turner's, 149. 

- zinc chrome, 152. 

ZINC chrome yellow, 152. 

- oxide, 53. 

- sulphate, 53. 

- sulphide, 119. 

- white, 126. 
Griffith's, 128. 




THE ABERDEEN UNIVERSITY PRESS LIMITED. 



C/fl 



RETURN TO the circulation desk of any 
University of California Library 

or to the 

NORTHERN REGIONAL LIBRARY FACILITY 
Bldg. 400, Richmond Field Station 
University of California 
Richmond, CA 94804-4698 

ALL BOOKS MAY BE RECALLED AFTER 7 DAYS 

2-month loans may be renewed by calling 
(510)642-6753 

1-year loans may be recharged by bringing 
books to NRLF 

Renewals and recharges may be made 4 
days prior to due date. 

DUE AS STAMPED BELOW 



51: 



A JUN I 5 2001 



FORM 



12,000(11/95) 



BERKELEY, CA 94720 



$ 



VC 40338 
U.C. BERKELEY LIBRARIES 




rp 



r 
4 

THE UNIVERSITY OF CALIFORNIA LIBRARY